Not only that, but you might not necessarily be able to fix it yourself either! You might be a developer who can learn to use Godot with GDScript or C# (just an example) and could get comfortable with its abstractions, but might have never worked on your own engine before and might not have the pre-requisite C++ experience not to make horrible mistakes.

Using an off-the-shelf engine is generally supposed to allow you to get by with a team of game developers, instead of needing another team of game engine developers. That's not to say that it's not very lucrative to have the latter for larger organizations, though I think that smaller teams have different concerns.