I think the main difference is youtube gives you access to nearly everything without a login while instagram does not.

Having said that there are some web pages allowing you to browse "public" instagram accounts outside of the instagram page but I guess they rely on tons of short lived bot accounts.

There was Barinsta which Meta DMCA'd down and threatened to sue its developer if Barinsta is not removed and discontinued. As is always the case, the developer got scared, had complied and these news spread across the community, so now there might be less developers who dare to publish an alternative, even less to put a lot of work into something that will be rewarded with threats and lawsuits. The reason why there's no alternative to Instagram is corporate arrogance. That is a common phenomenon at corporate giants like Meta.