Decentralised services always come out years later centralised services are launched and their only raison d'etre is not having the rules of those services.
Or, before then, do you still remember when email was this walled garden where everyone had an account with Electronic Messaging Solutions Inc.? It took decades until sendmail, exim, qmail, postfix, and so on came along, largely thanks to Google's help in breaking open the former monopoly, and people could finally host their own email servers!
Are you sure you aren't just saying "those decentralized services that happen to be built as replacements for centralized services come out later than what they intend to replace"? Well, yeah, of course they do. While those that happen to be invented decentralized first come out decentralized first. Also, I think email still is a decentralized service of today? And yet, gmail seems to be going strong in centralizing email, doesn't it?!