Its your boots/shoes, your shifting style/technique, or your settings, or even a combination of them. Most manufactures use the exact same rubber part on the shifters, and I have to remove and replace them all the time for powder coating. Its 50/50 half the people wear them out and half look new. I personally do not wear them out that badly that fast, but I dont wear boots and unlike alot of riders I actually adjust my shift and brake lever heights to match my riding position/style....Where as alot of people just ride the bike the way it came from the dealer and dont adjust things that are supposed to be a preference adjustment...Then you get people stripping out 2nd gear because their shift lever is too high and they always miss shift up shifts...
Id take a look at your lever height it may be too high causing you to put excessive pressure on it
Also if you ride into the winter season, the salt from the road can deteriorate rubber parts on your bike like that, it dries them out and they get brittle and fall apart easier...You can tell when the rubber starts to loose its blackness and get hazy and hard.