I think you're misunderstanding. Many of these companies, some with smaller budgets than Bethesda, create their own engines. Also, Bethesda has more than enough money to buy access to a better engine if they didn't want to create their own. Bethesda is not a small company anymore. They have millions of dollars at their disposal and have access to the best engineers in the industry.
Their only excuse is that they know we will buy the game regardless. They know their current engine is easily moddable, and even though they could invest the time money and resources necessary to create a new one that's just as easily moddable, they don't see the point despite the current engine being 15 years old and being pushed to its absolute limits.
If you're not having issues with your game, then that's great! But let's not pretend that a lot of the complaints against Fallout 4 aren't coming from a very real place, with crashes to desktop, people being forced to play in windowed mode in order for the game to function, framerate being tied to simulation speed, incredibly poor AI, the inability for many people to make a second character without the game crashing, and numerous other problems.
Bethesda allows this to happen. They're a very capable and lucrative company. This is their choice.