In my opinion, that's because healing is such a potential mechanic in the game and that's not just actual healers; that's heals in general. Herc self-heals from damage taken and considering our burst meta, that's generally a lot of healing. With that being said, even with all the antiheal items in game, most of them have either some condition of use( only works on basic attacks, etc.) or the ones like pestilence that reduce in an aura can't do much alone. Generally you'll need more than 1 antiheal heal to counter just 1 because they all work on different accords? playing against Hel as physical? well you'll need pestilence to decrease they heals while being around you but that's not enough. now you get beatstick to get her heals lower from her hits.... provided you can hit the slippery bastard. but even then thhose two items don't counter her heals well enough.they might inconvinience them but they are a complete counter. especially when she can get lotus crown, chrnos pendant to spam them so even if they're lowered she can do them more which is just as good.

When you think of the number of heals through items and abilities vs antiheal through the same methods we actually have less anti-heals to combat healing. a team full of guan chang'e, Herc, Hel, and Sylvanus could sustain in great order(of course provided they're decent in this scenario.) While the enemy team that combats them would need time to counter them considering there a like a handful of antiheal abilities most being ultimates so they have a longer cd, and the items are all items that'll take some good time getting to(other than weakening but then 2-3 people have to put down 900g just to shut dow the enemy and that's a lot of gold).

Even beatstick and divine take a bit of time to build, and healing abilities usually get to where they need to be between early to mid game.