Because not enough people do this earnestly, and many more do it maliciously (bot endpoints that lie, or provide significantly less information than people endpoints) or put it behind a business contract (yes, APIs), so the bots or agents can't trust it in general.

Also let's not forget that innocent sites suffering from floods of scrapers are actually the minority here - this is just a special case; the main reason for the tension is simply that most websites and businesses on-line rely on users wasting their time, and cannot abide any form of end-user automation. Their business plans hinge on their ability to force themselves on you.

Not to mention that it solves none of the rate issues. If the scrapers are hitting your site 10,000 times a day, adding markdown isn’t going to change that at all.