There are a few reasons for this.
1) Enterprise is mostly staffed with mediocre programmers. That's not meant to disparage anyone. Even great engineers will produce mediocre work if the incentives are aligned that way.
2) Enterprise problems have more to do with people management than writing the fastest and most cutting edge software. Frameworks can get hundreds or thousands of engineers speaking the same language for writing the boring code (which is the majority of code).
3) Enterprise companies often already have an established business model that works. They often have market dominance in some area and it's okay if their software is a little slower/buggier/etc. Game industry is no different.
4) Enterprise companies often have enormous scope that smaller handmade tools can't always cover. For example your average homegrown UI framework probably won't have accessibility in mind from day one.
5) There is no way to judge the quality of homegrown tools until it's too late. Our industry (like any other) has very loud yet very dumb people who convince others that their way is best. They build up layers of awful tooling/architecture that needs to be rewritten down the road. Frameworks have at least some form of validation.
I used to believe the above was fixable. Now I know better and just stay away from big companies. Work is more fun and fulfilling that way :)