This is because most of them are just using open sourced code from github. You can do a search on youtube and find videos showing you how to create clones of various sites for free and with an hour of work and basically no knowledge you could have your own NFT marketplace website like rarible just by following along with the video. Now I'm sure OP isn't just going to change some names on open sourced code and charge you money for it as that would be seemingly a bit fraudulent without the proper notification prior to accepting funds, but if you don't mind a cookie cutter NFT contract, youtube/github is certainly a cheap and easy option. If you want a more customized solution with special features, hiring a coder might be a good idea.