That's not quite fair to compare, a modern computer OS has a lot more features than something like Amiga OS.

Say for example, you have a folder full of movie files. The difference in computing required between just displaying a list of the files in there vs interrogating each file, generating a thumbnail on the fly and possibly sorting based on some metadata inside the file is probably more than an order of magnitude.

Not to mention all the extra stuff you have for security and stability when you are running so many applications at the same time.

You also have the classic trade-off of developer time (and therefor time to market) vs computer time.