Also, because no one uses SSDs and those definitely don't have a limited number of writes. That said, they could make it smarter ("We see that you're using an SSD but you have some free space on this other disk. Should we use that for cache instead? Click here to read more about SSD write cycles...") or just cache in RAM only.
Even with a page file on an SSD I haven't had a problem over years of operation. Granted, this is anecdotal, but wear levelling wouldn't exist if it made no discernable difference. Windows disables defragmenting on SSDs, but from what I gathered that's not so much because of write cycles but rather because it makes no performance difference and thus is time unnecessarily spent.
That being said, making the algorithms smarter should probably help, e.g. only caching in memory if there is an SSD and just store frequently-used sites' resources on disk which might be a viable compromise.
