Yes, that reason is more money and better exposure. Nearly all of the examples you are giving are players transferring amongst power schools. Do that same exercise for great players who transferred from a P4/5 to a G5.
This system is an upside down funnel, the lower you are in conference/money/brand/whatever, the more opportunities above you players will have as better options. If you are a top 20 school, why bother even trying to recruit anyone other than the best of best high schoolers when you simply can acquire already developed/proven talent by offering…wait for it…more money and more exposure.