You will see a fraction of the traffic that somebody doing the same thing on those platforms will see.
They try to hand wave it with build a tower and bring them back to your site but that rarely works well.
I need to create an account to use your site has a significantly higher bar than I hit subscribe to see your next video in my feed.
But they are small business owners. They make their living entirely based on digital visibility. They need to get their message out to where the eyeballs are. They may try to get people to subscribed directly to their e-mail newsletter, but that's not enough. Most people find them on Instagram, Twitter, etc. If they delete those accounts, as they would like to, their business will be in deep trouble almost immediately.
Web discoverability has had the same dilemma since its inception. People only remember and actively engage with a few things. A search engine, some media platforms, some communities they are involved in, etc. If a link appears in one of those places it's extremely visible. If a web page does not show up in one of those places, discovering it is next to impossible. What are they going to do, guess the URL?
How can someone get some amount of visibility on the web without putting anything in anyone else's kingdom? Even someone following the POSSE model (post on own site, syndicate elsewhere) is extremely dependent on the elsewhere if they want to be visibility. Without the elsewheres to syndicate to, they will build an empty and isolated kingdom.
If you do your work, it's not hard to get good visibility on Google and other search engines. The key is this: If you're selling product X or service Y, you need to make your website the very best resource imaginable for information about it and with an as easy purchase process as possible – with good terms to boot.
But most small business owners are completely uninterested in that, and instead spend their days spamming social media and paying for ads to bring visitors to their website that turns potential customers away instantly.