Reason is Speed. Also a process can grow while it runs, like k-meleon. Or spawn new processes while it runs like chrome.exe. Cache memory is faster than RAM which is faster than disk.
So things get put in cache where cache is available. Ideal is if CPU is executing things in cache. Second to that is if it's executing things in RAM.
CPU never processes things on disk, it doesn't address things on disk, it only addresses RAM locations or no doubt, cache locations. Or locations that read/write to devices.
It's not like everything even everything you might want to run is swapped out to disk and then back to RAM. Only Things not currently being used and not likely to be used soon are preferred when swapping things out, so as to free RAM up for things that are more in use.
Web browsers have a cache too, a disk cache! They store things locally on disk, rather than fetch from from online. There are web proxies that you can run locally that store a website locally and if it changes online then it will fetch. This enables you to browser much quicker. (though I suppose you can run into the problem of a website not specifying properly that a change was made, and you get stuck loading an old copy from your local disk cache) but the idea is that it's faster. In that situation the quicker area is the local hard disk and the slower area is the remote/online area.
You do this kind of swapping in life, with things you need near you, and things you don't need in storage. You don't put everything in storage/garage arguing that there's loads of room in storage/garage. It'd be too slow.