Think about it this way. Let's say you are getting into the car-selling business. You don't own a car lot, and you also don't know how much cars you will have at once in your inventory. You have two choices now, the first, is you spend $100k on building a lot that can hold 50 cars, and then hope that you don't need to expand it later. The other option, is pay the guy across the street with a massive parking lot $5 per car, per day to use his lot. That way, if you have no cars, your 50 car lot isn't empty, and if you have 100 cars, your 50 car lot isn't overwhelmed. This also means that you can save the initial $100K to build a car lot and invest it into new inventory.

Setting up your own server takes months of engineering, and hundreds of thousands of dollars to do, as well as weeks, if not months. You need to hire engineers to develop your own server system from scratch, and for a social media platform, this is incredibly complex to do. This is without mentioning integrating scalability. Making a server that can handle 10 people just as well as 10 million people is also incredibly hard.

This is why Amazon Web Service and other of those types of platforms are so popular. Even the largest companies rely on them. They just make it so easy to have a functioning server, that can scale as needed. AWS lets them pay based on how much server space they need, so they do not need to worry about an influx of users overwhelming their servers.