its harder to hire new devs if engine is built in house, because no one outside the company understands how to use said engine unless its open for the public to use. thats the biggest drawback of in house engines (other than the increased develepment life cycle to develop one)
its why for example, many 3rd party ports/remasters of old games use unity for example.
Using an inhouse engine makes sense only if you can retain a lot of talent. or have several projects that use it as a base.