Go ahead and make a Twitter clone this weekend. There are plenty of tutorials. Then let's assume you go viral and now your little service needs to scale. Then the email and issues start to come in. Now you have to balance fixing bugs and tending to customer support. Oh, are you going to outsource all cloud services or hire a 200k+ a year SRE to scale it out? One wont be enough you will soon find out. Did yo do your research on compliance? Are you sure your app is legal in every country it serves? Pretty soon you have a team whose salaries + benefits are costing you several million dollars a year. And you only have 10 people.

I could go on and on. Yes, creating a twitter clone is trivial. Scaling it and tending to real customers is a completely different story.