4 lane highway vs 1 lane dirt road. While combining the two makes the number of paths greater the paths themselves are not equal (my equal cost load balancing remark). So, if the router is not taking link cost into account, which it is not, you'll actually have worse performance over all of your connections because the router will try to balance them equally on session start. This means that 50% of the time you start out going to your neighbors connection and fail or go slow. It's actually more advantageous to wait for a spot on the wide, fast link than it would be to take the alternate path.

If the router was taking into account link reliability and speed (overall cost) it would only give a small percentage of connections to the worse link and only if the main link was saturated from a bandwidth perspective. None of this is happening by round robin load balancing which means more of your connections are worse. This is why routing protocols that have these features exist.