The difference is that there are fairly objective measures of value that you can apply to equities. We are probably a little overpriced right now based on P/E ratios. But unless something world shattering happens, we can be pretty confident the S&P wont be at 50% of what it is today. If for no other reason than because you actually have an ownership stake in the companies.

But is there any coherent explanation for why bitcoin should be priced at 2500 dollars rather than 2.5 dollars? or 2.5 cents?

The US dollar is a bad example because it's not supposed to be used as long term store of value at all. The inflation is purposeful and much more importantly fairly predictable. A bank can lend me 1,000,000 dollars to buy a house because they can be fairly confident that inflation is going to be ~2%.