Investors believe in the currency's future and keep it as a long-term investment.
That's for the big investors but the small investors slowly disappeared from the ethereum platforms caused by they didn't wanna waste their money just to pay the miners.
The long term investment is only suitable for the whales now.
Any major coins are capable to be used as the store of value based on the various factors like development, fundamental, community support, adoption rate and many things can't be written here.
There are still many small investors holding ETH for long, as long as you don't make money transfers, you don't need to worry about fees.
While all the major coins in the market can act as a store of value, ETH seems to be one of the best coins for doing so.