Mainly because there's not only just development team in a "website".

• If a company wants to have this application be connected to a server, a server cost money to buy/host and run, so now we need account department.

• Hosting a service 24/7 would means we would need people to maintain the server so it can stay up for 99% of the times.

• Now we need our source of income for paying our staff, We would need someone to deal with advertisement and such.

• Alright, we want to make sure our "website" to not get sued by someone for stupid reason, we would have to hire a legal team to deal with that.

• What if someone encounter some issue while using our services, our development team are way too busy for that, customer support it is.

• Hire new employees, too busy to be dealt by anyone, HR department it is.

• Our "website" will store our user's sensitive data, better to hire a dedicated security department for that.

and there's more, all of these can't just simply be dealt by just 3 to 10 people.