Mainly because there are only a couple of core classes of wormer active ingredients, and when you blanket treat (and sometimes people under dose by mistake), the worms eventually become resistant to the core chemical. This is becoming a major problem in practice not only a theory, and since there is no obvious totally new classes of wormers, if we all carried on as in the past just worming as routine, then in the not too distant future we will have no effective wormers left to use on the animals.
Horse owners for decades routinely wormed (and if you didnt you would be considered very irresponsible), but as resistance problems have grown, so attitudes have changed and 'intelligent worming (worm counts plus targeted worming when and if needed) is now becoming the norm.
The other reason is these are pretty nasty chemicals to be having spread over your land and very harmful to the beneficial bugs eg dung beetles that magically make dung disappear and stop fields becoming one big dung heap. They can also find their way into the water supply.
The need to worm as routine may be increased if there is a high stocking rate but for animals farmed extensively and kept for any length of time, worm counting plus targeted worming is far preferable to blanket chemical use.
