1. Their use case is kinda different. The servers mostly run heavy CS research related stuff. E.g. they might have heavy CPU load and heavy traffic between they servers but they have less often heavy traffic to the "normal internet" (if they have heavy traffic to the outside it's normally to other research institutes which not seldom have dedicated wire connections).
2. They might run target specific optimized CPU or GPU heavy compute tasks going on for weeks at a time. This is really expansive in the cloud which is mostly focused in thinks like web services.
3. When they don't run such tasks in the research groups they want to allow their juniors to run their research tasks "for free". Which wouldn't work with a payment model as done in the cloud.
4. They don't want to relay on some external company.
Also I'm not sure are there even (affordable) cloud systems with compatible spec? (like with 4+TB of RAM, I'm not kidding this is a requirement for some kind of tasks or they will take way to long and requires additional complexity by using special data structures which support partial offline data in the right way, which can be very costly in dev time)??