Why did my tires wear so quickly?

Cheap tyres.               

Some people are lucky.

https://www.cctyrespenrith.com.au/

I think it might be because you sanded them to be smooth, Sandra 

bad front wheel alignment 

Add low air pressure to anonymous' list.

Poor front end alignment.

Not rotated tires often enough.

Vehicle has rear alignment or suspension issues causing it to "dog track'.  (google search "vehicle dog tracking" if you don't know what that is.)

Tires not properly balanced.  (would probably also notice some driving issues with this problem)  

Bought poor quality tires - or dealer cheated you and gave you a lower quality tire than you asked for.