I do have a theory about this.

There are passengers who care about where they sit, and passengers who don't. The passengers who do care usually know to use MMB to pick a seat that they like. They are also the passengers who are more likely to be upset if they are put in a seat they don't like.

If you give out the best seats to the first to book, you risk giving some of them to passengers who don't care, and wouldn't have noticed if you'd put them in a rubbish seat. This approach reduces the supply of good seats for those who do care.

If, however, you give out rubbish seats at the start to everyone who books, those who do care about where they sit will go to MMB to pick a good seat. That system is more likely to satisfy the passengers who do care (because those who don't care won't change their seats, leaving the good ones for those who do care) and who know how to use the tools made available by the airline to make the journey more comfortable for themselves.