You build a Twitter. Profiles have posts, posts can have images, etc. It's very easy to model the database.
But then how do you make money with it? Now you need to build a separate system for advertising? Or do you want to sell subscriptions? Which means you need to build a separate system to handle payments. This is usually the big one, because when you handle money, what happens if there is a bug and you charge someone without delivering anything? How do you prevent fraud? How do you handle disputes?
Someone posted something illegal. What do you do in this situation? Do you call the police? The FBI? What kind of data do you give the authorities? How much data SHOULD you have been logging in the first place in case something like this happens?
One user doesn't like you so he bought a botnet to DDoS your website. How do you handle this? Are they mass posting? Mass creating accounts? Is it possible for them to exhaust all the usernames possible and then nobody can create an account anymore?
Your website is online but if the server blows up you'll lose all the data in the database. You need backups. You need a system to ensure the backups are actually working. But then some guy from the UK said he wants his posts all deleted. What are you going to do now, because his posts are also in the backups, and you don't want to touch those.
Trolls are posting things against the ToS. Who handles these things? Shadowban? So there needs to be a shadowban system? Moderators? So there needs to be a moderator-only section of the website? Should this be integrated with the main website or not?
Then you look at this horrendous mess of 6 paragraphs and you think back about the first paragraph that already did everything you wanted from Twitter. All these other systems, most of the work, and all you actually wanted was the first paragraph.
What actually happens in a startup is you encounter these problems one at a time as they arise.
Use stripe, cloudflare, whatever the legal equivalent of these stuff, S3.
Yes they might take most potential profit, but you'll also not have a huge payroll.