Why lip implant is not a good option? 2 doctors suggested them but don't recommend them

I have extremely thin lips. in 3yrs Iv done 12 syringe Lip fillers, 4-5 different Products such as Juvederm ultra , restylane kyss and couple more. I've consulted four doctors in two cities, each lip fillers disappear within two weeks, as if I never got them. This has been extremely frustrating. I suspect my fast metabolism is to blame. Two doctors suggested lip implants as an alternative, but they don't recommend them. Why ?