Welcome to the site !!
The reason is the valve seat is harder than the valve face , and as it is slammed shut if there is even a trace of dirt on the face it acts like a grinder , and cuts away the face of the valve , as it wears your gap decreases , to the point where it zero's out , eventually to the point where you can't find a shim small enough to fit the space , because as the face wears , the valve moves closer to the seat
Basically once the valves start moving they are on their way out and will need to be replaced along with the seats being recut or replaced , mine went at about 6 months from new (in 06) from dirt getting past the filter , so i went to Stainless Steel valves instead of Titanium and have not had them go out of adjustment since , this is what most people do , you can buy a replacement head ready to go with new valves and seats from here , or you can find someone local , (or have it done here) replace the valves and cut the seats , just make sure they are proficient in doing modern 4 strokes , they are different than cars and need special tools made specifically for doing these engines , it wont be real cheap , but once its done you can ride for a long time without any issues ....... The key is to make sure the air filter seat is clean and sealed at the face , the dirt gets past the filter if its not oiled or seated properly and it takes out the valves , its a expensive way to learn filter maintenance , but its a real common issue
This will give you a idea of cost , for doing the valve job , or just replacing the head
https://www.crfsonly.com/catalog/index.php/cPath/258
Sorry to give bad news , but basically , you will need to either replace the head or replace the valves , or you could try a used head on eBay , but may end up with the same issue fairly quick and be out more money ....