Because from an infrastructure and cost standpoint, it's incredibly inefficient. And it's not like the current system guarantees speedy transport for guests. If it meant waits were no more than 5 minutes then it would perhaps be ok, but it's still not uncommon to wait 15 or 20 minutes.
WDW has around 30 hotels. The point to point method means each of these hotels has at least 5 bus lines (the 4 theme parks and DTD). That's around about 150 distinct bus routes crisscrossing WDW, varying on crowd levels and whether some hotels (like the All Stars) are sharing one route or if each has its own. Let's say there are (average) 2-3 buses on each route to move people about. That's 300-450 buses and upwards of 900 drivers (multiple shifts, remember) just to move people from their resort to the parks. The same movement of people could be done with fewer buses and with shorter waits if it was reorganized to be a coherent network rather than a bloated shuttle system.
If Disney insists on only using buses, then instituting a network of "express" bus lines with dedicated lanes down the main roadways of WDW with transfer points to "local" resort buses makes infinitely more sense than it does to keep trying to maintain the point to point system that is currently in place. If done properly, it would mean less overhead, better use of resources, and (I think) shorter overall waits and higher overall bus speeds.
