and they are generally pretty crap? easily lost, forgeable, destroyed, manipulated, unrecognized.
typical certs require a centralized "Certificate Provider" who upholds their value, for example a major auction house might distribute a cert when selling a painting. in crypto art there is no centralized authority that maintains the veracity of the tokens as the provenance and ownership lies in the distributed ledger.
for those who wish to achieve similar goals as a digital cert but with more cryptographic robustness and less reliance on a centralized actor, NFT is a fine approach. that it has sales and market mechanisms built in is a bonus and probably the reason for its booming success as a new digital art market.