balancing/alignment/camber/toe adjustments, absorbers possibly even spring, badly damaged driveshaft/tie rod/steering rack/wheel bearing. check all those, it'll be either 1 or a combination of the items i mentioned

if you're on a proton then it's only few hundred ringgit. other cars especially conti ones, yeah prepare to break that piggy bank

5-15k km, not months.
time ages all tires, however tread reduces the more you drive.

wrong rotating method
and yes as mentioned, please spend time to check your tires oftentimes, and not wait until they get uneven wear/balding and start to panic