This is the sort of thing that sounds like it wouldn't work until you realize that it's literally Microsoft Flight Simulator, but with cars instead of planes.

Here's a fun fact about Flight Simulator. Years back I used to work in a computer store, and we'd do custom builds. My best sales, by far, were always to the smartly dressed guys in their late forties who came in looking for a gaming PC. You'd ask what kind of games they play, and they'd tell you "Just Flight Simulator." They really meant it. They were building a gaming rig for one game. And then they would drop the most heinous stack of cash to build a gaming rig that would make the gods weep in envy. Massive towers with double or triple GPU, maxed out RAM, best CPU on the market. Liquid cooled? Sure, fuck it, why not. Their parents built model trains. These guys had Flight Simulator.

As a franchise, that game has 22 million sales. The Forza franchise has 16 million.

All of which is to say that I do think there's a missed opportunity here. I hate racing games, but I'd play an open world driving game and I know at least one friend who would play it with me. Cruise down a mountainside in Italy, navigating hairpin bends with the hammer down and a beautiful sunset in the distance? Yeah, fuck it, sign me up.

This sounds pretty similar to what I did back in Test Drive Unlimited, specifically the first one not the second. TDU2 might also be able to do the same thing but my brother sold my copy before I could form much of an opinion on the game. I saw one other person in the comments mention TDU2 but saying that they have "racing games like TDU2", so it seems like they didn't get the same experience I enjoyed out of TDU1.

The main similarities I see are that in TDU1 you had very pretty cars on a very pretty island with a large number of roads to go up, down, over, and between. The roads were a means to facilitate races and other missions as much as they were there to allow free roam. Missions and races could only be started if you had found their start point during free roam and you could teleport to any part of any road you've previously traveled on.

The total quantity of roads you've driven down is also a metric that the game tracks and offers an achievement for 100% completion. The game's minimap would also show untraveled roads as grey and traveled ones as blue so you could always just roam about and try to find new little roads and sections that you previously missed.

The progression of the game was through the missions and races which got you money, but free roam is easily the mode that I spent the most time in. Also you'll note I keep saying missions and races. I no longer remember most of the mission types but I do remember there were a few and they were markedly different from being races with extra steps/restrictions. The primary one I remember was delivery missions where in game characters would hire you to deliver their vehicles, then you'd get money based on the value of the car and it's condition by the end of the delivery.

If memory serves then you would also get a 50% bonus to your paycheck if the vehicle made it to the end without going off-road and without a single scratch. I specifically remember these missions not being repeatable meaning that any money you could earn from them was one-time and if you didn't earn the bonuses then you wouldn't get a chance to earn that money again. Most of the time that wasn't too bad, but considering some of the missions were fairly high value for the early game, it (especially then) was pretty beneficial to be very gentle with those delivery missions so you can buy your first few cars.

The experience you're describing reminds me of what I most valued out of TDU1, being able to just explore around a large world while driving in nice cars. That being said from what I understand your idea wouldn't have much in the way of progression. For me, I don't think I would have started exploring the world had there not been an in game incentive to do so. There were plenty of times I was exploring just for the sake of exploring (like the many times I went riding through the countryside off road, or tried to ride a ramp into the airports), but I'd still say the vast majority of the times I was exploring it was for that sense of progress I got from finding more roads I hadn't yet traveled, finding more missions to complete, and trying to earn that 100% completion achievement.

If at least some of those aren't in your hypothetical game I'm not certain I would get much out of the experience other than a pretty world existing for the sake of being a pretty world. And perhaps that would be enough for some people, but I don't think I'd get much out of the experience.

No sounds boring af

This sounds like the Train Simulator of driving games, which I'm sure there's a market for. I think it could have more mass appeal without compromising the vision if you included a set of in-game goals like visiting various landmarks, obeying (or disobeying) road rules, or whatever else.

As someone else said, you can do this in Forza horizon. Make a custom car, tune it yourself, then rip around the map blasting music.

Yet it would be pointless and yes it would get boring. Pointless it can get away with easily enough, but boring, never.

In GTA and even NFS open world games, players would sometimes play by the npc traffic rules. It's fun and different, in small amounts. But making a whole game about it? Rockstar's betting on Roblox style development for GTA6. Which means someone's bound to make a mod for that game doing exactly what you want, but with the foundation already built.
It's an uneven competition.

So to succeed, this kind of game has to have its own shtick. To make it not boring. A way of showing off perhaps?
Reactions to activities, which reflect a form of growth or change.