Lots of people block them because they’re really really annoying. (Also a major security risk that slows down your browsing experience, but I doubt that’s the main reason.)
Because adverts are executable (who thought that was a good idea?), they also get used for cryptocurrency mining. Really inefficient cryptocurrency mining, but still.
Because they cost money, there is a financial incentive to systematically defraud advertisers by showing lots of real, paid-for, adverts to lots of fake users. (See also: adverts are executable. Can one advert download ten more? Even sneakily in the background will do, the user doesn’t need to see them.)
Because of the faked consumption (amongst other reasons), advertisers don’t get good value for money, lowering demand; because of lowered demand, websites get less money than they would under an efficient system; because of something which seems analogous to hyperinflation (but affecting the supply of spaces in which to advertise rather than the supply of money), websites are crowded with adverts; because of the excess of adverts, lots of people block them.
What if there was a better way?
Cut out the middle man, explicitly fund your website with your own cryptocurrency mining? Users see no adverts, don’t have their attention syphoned away.
Challenge: the problem I’m calling hyperinflation of attention (probably inaccurately, but it’s a good metaphor) would still apply with cryptocurrency mining resource supply. This is already a separate problem with cryptocurrency mining — way too many people are spending way too many resources on something which is only counting and storing value but without fundamentally adding value to the system.
Potential solution: a better cryptocurrency, one which actually does something useful. Useful work such as SETI@home or folding@home — if it must be a currency, then perhaps one where each unit of useful work gets exchanged for a token which can be traded or redeemed with the organisation which produced it, in much the same way that banknotes could, for a long time, be taken to a central bank and exchanged for gold. And the token could be redeemed for whatever is economically useful — a user may perform 1e9 operations now in exchange for a token which would given them 2e9 floating point operations in five years (by which time floating point operations should be 10 times cheaper); or the user decodes two human genomes now in exchange for a token to decode one of their choice later; or whatever.
A separate, but solvable, issue is that the only things I can think of which are processing-power-limited right now are research (climate forecasts, particle physics, brain simulation, simulated drug testing, AI), or used directly by the consumer (video game graphics), or are a colossal waste of resources (bitcoin, spam) — I’ll freely admit this list may be just down to ignorance on my part — so far as I can see, the only one of those which pairs website visitors with actual income would be the video games… but even then it would be utter insanity for the paid customers to have their image rendering offloaded onto the non-payers. The clear solution to this is the same sort of mechanism that currently “solves” advertising: automated auction by those who want to buy your CPU time and websites that want to sell access to your CPU time.