

Did they actually build their own alternatives that anybody wanted to use,
Right wingers functionally excluded from major platforms more or less until Musk bought Twitter actually did create their own spaces. Several of which got made functionally inaccessible from mobile devices, or removed from search engines or otherwise actively prevented from growing organically, many of them eventually being quietly put back after any buzz around them had died down. Think Gab, Parler, various .win sites that are reddit-like, etc.
Hell, at one point the largest Mastodon instance was literally one such space that had had it’s own app banned from the app stores for being an uncensored far right space, and then several of the major general Mastodon clients made their clients refuse to connect to that specific instance at the client level as well. If you’ve ever seen a Mastodon client on say the Play store with negative reviews saying that it couldn’t connect to the largest instance, that’s because at the time the largest Mastodon instance was technically Gab.
…but they did, and were broadly prevented from growing organically. See Parler and Gab.
Gab actually changed it’s underlying backend twice. They started with their own thing, Google and Apple cut their app off from the app stores so they switched to something that was a common protocol rather than a dedicated app and moved to Mastodon as a way to route around censorship. Most Mastodon servers promptly defederated them for being right wingers, and several of the major Mastodon clients built in client level mandatory blacklists causing their client to refuse to connect to Gab (in some cases Gab was the entire blacklist) - you can see which ones because they have some older store reviews about “refusing to connect to the largest Mastodon instance” because for a time that was Gab. Later on, they moved to something involving crypto somehow, I’m not really sure about the details.
Parler appeared, got big fast, got kneecapped by Google, Apple and Amazon and by the time they recovered had lost most of any momentum they ever had.
They even produced their own Reddit-alike, spread across several .win domains - that was a thing that happened in response to the biggest Trumper community getting banned from Reddit. That original community migrated to TheDonald.win and later rebranded as Patriots.win.
Truth Social is basically what Gab/Parler wanted to be, but big Tech doesn’t want to fight against Trump because he’s exactly the kind of petty asshole to fuck them over for considering it. Truth Social is of course Mastodon under the hood with federation disabled.
But if we presume fediverse apps are a protocol rather than a proprietary product (which is usually how they are sold to users, making analogies to things like email or the web as compared to a proprietary system) why would you need to reinvent the wheel? No one is arguing that we need an exclusively communist alternative to email or the web.