why doesn't it really work and why do people come back and do it anyway? It's because cutting off is the most basic non-solution, that doesn't actually try to tackle the problem or fix anything. (Perhaps less so for 'managing use and having time limits', but definitely that for just 'cutting completely off, down to zero' approaches.) In a digital and online world, cutting people off is inhibiting people's communication. It's just plainly unfair. People need the ability to communicate, and venues to socialize in, at any age. And, it is not given that just because someone's 'technically of adult age', that they're gonna have some kind of understanding about digital spaces and how social life can play out within them. Just being in a space, or even having some experience within it, isn't necessarily gonna give explanations, answers and solutions. It is just a thing that's worth explaining and giving guidance on, whomever they may be.
Perhaps one useful and applicable distinction that can be made, is between 'interpersonal communication' (in apps like messengers, directly talking to people you know) and 'posting something on a platform' (in apps that are more like "social media", where posts are available to a broader public). Another distinction could be made for 'private platforms/spaces' (like discord) or 'limited/like-minded/focused spaces' (that aren't just the most general platforms, but something bit more specialized for some kind of content or audience), and the bigger platforms/spaces (that are basically just 'you're out there with everyone'. it also could be pointed out that 'having a semi-private group on a big platform might not be too bad - but that it might be still out there and accessible, so there's things to mind'. or, hell, explain what comes with posting on a big platform and what precautions should one take). Explaining the differences and outlining where and what someone would be doing, could help, in a practical way that won't be just 'you can't do anything now'.
Cutting off messaging isn't something that should be done, in my opinion - it's too drastic. it's basically saying, 'don't talk to people/you don't get to talk to people', and those people are probably gonna be the most familiar/important people to them. Cutting off social media, could be done, but it's really worth talking through and explaining the difference between 'publishing something for the whole internet and hanging out in that broad of a space' or 'posting something for your friends group to see, and hanging in a private, familiar space' (and perhaps, enticing them to grow and enrich their own social circle and seek something there, not just from random people on big platforms). At the end of the day though, cutting off and prohibiting something can be just circumvented, as people might just do something anyway, and if they don't have the 'why' explained to them, it might be even easier to ignore the limits without much concern. With guidance, that stands on its own, maybe it could be less so.