Unreal licensing is crazy compared to Unity. A % of revenue is millions of dollars potentially. You cant compare that to the fixed opex per developer seat with unity. It’s a strategic design for a company to use one or the other. So far in all companies I’ve worked for and have consulted for attribute their choice to cost and ease of hiring.
To underscore the sibling comment: at the point where a "% of revenue is millions of dollars" the company can easily afford to invest in an in-house solution if it's that important for their revenue goals. The tradeoff is hiring a sufficient number of skilled game engine programmers to make that work. It's not cheap.
Basically if you are big enough that you can make your own engine the engine also has to be usable by non programmers making the actual content for your game.