Because mining and the Proof-of-work algorithm can't become that fast, I believe. Eth is ridiculously fast with 13s blocktimer considering it still works through PoW and even so it sometimes leads to orphaned blocks due to latency and other stuff.

8mb is only the beginning, if adoption gets so big that they need more they will surely increase but that also means that the wallet will start getting really big - yet not too big for casual harddrives to handle considering technology is becoming better and better every day.

I mean, a 1 TB harddrive today is nothing considering how big they will become in a couple years. Bitcoin wants to keep PoW mining due to its safety against 51% double spend attacks and having to have real machines wasting electricity to make sure no one can cheat it although Proof-of-Stake such as Steem is has already proven to work really well and doesn't even come close to foul players trying to cheat it its still undervalued and overlooked, in my opinion. There are some others like it like BTS and Lisk and even Eth is planning on going PoS in the near future so we will have to see how the money starts shifting when that becomes more common.