The software development culture on the games industry is to make use of the best way to squeeze the graphics performance of a given piece of hardware until the last drop of juice.
This is an industry that was born from rewriting their whole games for each type of gaming platform they wanted to sell games to, in Assembly. In many cases using a dumb hexdump editor connected over a serial cable.
Having an abstraction layer over all graphics API is a minor inconvenience that can be easily done in one month.
Plus it is something that is only written once for a specific engine.