No magnetic confinement fusion reactor in existence has ever generated a positive output. The current record belongs to JET, with a Q factor of 0.67. This record was set in 1997.
The biggest reason we haven't had a record break for a long time is money. The most favourable reaction for fusion is generally a D-T (Deuterium-Tritium) reaction. However, Tritium is incredibly expensive. So, most reactors run the much cheaper D-D reaction, which generates lower output. This is okay because current research reactors are mostly doing research on specific components of an eventual commercial reactor, and are not aiming for highest possible power output.
The main purpose of WEST is to do research on diverter components for ITER. ITER itself is expected to reach Q ≥ 10, but won't have any energy harvesting components. The goal is to add that to its successor, DEMO.
Inertial confinement fusion (using lasers) has produced higher records, but they generally exclude the energy used to produce the laser from the calculation. NIF has generated 3.15MJ of fusion output by delivering 2.05MJ of energy to it with a laser, nominally a Q = 1.54. however, creating the laser that delivered the power took about 300MJ.
Another big factor is that every plant is effectively a completely custom design. Because of how few nuclear plants are constructed, every new one tends to incorporate technological advancements to enhance safety or efficiency. The design also has to be adapted to the local climate and land layout. This makes every single plant effectively one of a kind.
It also tends to be built by different contractors, involving different vendors and electric utilities every time. Other countries have done better here (e.g. China and France) mostly due to comprehensive government planning: plopping down lots of reactors of the same design, done by the same engineers. Although these countries are not fully escaping cost increases either.
You are completely correct that regulation is also a big factor. Quality assurance and documentation requirements are enormously onerous. This article does a pretty decent job explaining the difficulties.