radial ball bearings work excellent for radial loads (perpendicular to the shaft they support, but usually quite poor for axial loading
for that, if the tractor is often used on hillsides or ditches, that can reduce the bearing's useful life. Or frequent full speed turns. Also, foam filled tires, very heavy. Basically if you can avoid side-loading the tires, you can extend the bearings' lives.
Also, if you get the numbers off of the bearings, you might be able to do some digging to find a bearing that is more suitable for side loads than the factory replacements are. The factory doesn't really skimp too much on them but they also don't really go overboard either, they're engineered to last a while, but they also have to keep costs in mind and they know that there are going to be instances where some users will find the weaknesses.
They could build a perfect tractor, but it would be bigger than your house, weigh 25,000 lbs, only have 30 hp, and would cost probably $150,000. and nobody would buy it.