What level of knowledge do you have about how the Internet works?
1.-Itch contracts the server service to another company. If the other company fails, the Itch staff cannot kick into said company's data servers, point a gun at the technicians and force them to repair the server.
If your internet operator fails one day. What can you do? Do you open the junction boxes on the street and try to repair them yourself or wait for the company you pay to send a technician who can check and repair the problem? It is similar.
2-Of course they can, but in the years since the Internet has existed, it is much more expensive and complex to build and maintain your own data-center VS hiring the services of a specialized company. And I anticipate your question. Normally this model causes your services to be down for longer than when you hire the services of a specialized company.
3-I wouldn't know what to tell you, I haven't seen Itch be down longer than other companies of similar size. It is true that Itch hosts more than a million independent games, but only about 25,000 paid games, so as a billing they must be far, far below monsters like Google, Apple, etc. If you have the income of Google, you can pay for more redundancy than a small company.