Because it is just paper, point in time. Pick any 2 goal posts of time to determine whether you have a gain or loss. How meaningful is that?? Are you down from March or from buying 5 years ago or from a week ago?
Consider buying 100 shs of Stock X at any price. Price goes up, you have 100 shs, price goes down, you have 100 shs. Where is the loss??
Only by selling those 100 shs do you make anything "real". Everything else is just normal market gyrations usually having nothing to do with the quality of the company you own, and not stealing shs from you. Adding, maybe, via dividend reinvestment.
I have super long haul mindset, these things are very easy for me to tune out with decades of experience. Being a dividend investor, market bottoms are welcomed because for me, more shares = better, no matter the price.