Ultimately though it is all about time efficiency, solo or small teams have little manpower and since they don't even know if their first, second, or fifth game will sell any at all it may not make strategic sense to spend months reproducing common engine features in order to save a hypothetical percentage of revenue that they may never ultimately make. You need to test the gameplay concept as cheaply as possible.
Creating a game engine from scratch is FUN, and if game development is a fun hobby then full steam ahead. If however it is a business that you plan to make money on, you're unlikely to recoup the time investment into a bespoke engine just to test the market for your games. If the game turned out to be successful you could always piecemeal replace the commercial engine with bespoke via updates (inc. using money to hire people from the success).