Well, mostly because, generally speaking, the idea of a very limited BUT rechargeable resource (and this is a key point, it isn't "infinite potions" but "finite and rechargeable") is clever game design.
It both encourages the player to use everything he has while preventing the risk of the player just gathering too much and cheapening the challenge.
As already pointed it worked very well in Dark Souls with the Estus Flask system.
Now, the complaint here it seems to revolve mostly around not fitting the lore and not being particularly realistic.
Both complaints I can agree with, by the wya, even if personally I don't value them too much.
But at the same time you are "cheapening" the whole thing because the potions are available the WHOLE TIME, whenever you want.
There is no preparation (collecting ingredients, brewing the ones you needed), you ALWAYS HAVE THEM IN YOUR INVENTORY when having meditated (which is btw. incredibly annoying to have something refilled in my inventory when I only did it once and do not WANT to use it anymore because I don't like the potion and it takes up inventory space (and yes, there were also such potions in TW2), I mean come on).
Look, I tell you how I sometimes did it:
I am in the game, I took several quests, am about to go out and fight some monsters. I start meditating. Most of the time you have enough monster-parts anyway to create most potions if you are an active player and do side missions. I start brewing my potions, make staples of a specific one (5 or 6) while ignoring other ones (which I do not like to use). I begin to think "Do I need cat? Nah, no dark areas around and no caves there, but lets make 1 just in case", make this potions, make that one. Okay, let's go. First monster encounter. Kick their asses. Meditating, taking swallow. Going on, picking up herbs and killing monsters along the way (new ingredients already collected that way). Going on... okay, coming close to an area with lots of monsters, it might get rough (probably not, but hey, this is a role-playing game, so I'm in the role: I'm Geralt and if I don't want to die I better be prepared. Meditating, taking potions. Let's get the fun started.
It is PART of my experience, part of my role-play. And I just can't say how incredibly I will miss that. That's just how I feel.
Hoarding is a tactical play. If you hoard and don't use you are stupid, sorry. And if people don't use their potions it's probably because the monsters are not enough of a challenge to require potions. Solution? Make monsters harder....
And to be honest, I can not understand people telling me getting ingredients is a chore, actually they just fall into your bag as you kill monsters and walk in the world since there is normally about 20 times the material you need to make your potions.
On a side note, I'm still not sure how meditation is supposed to work (see @Broghain post just above).
Are we going to have fixed usable firecamp? Will you be able to rest at will but there will be areas where you can't?
Not sure if we have any official statement on the topic.
Would be even worse if we could only meditate at fires.....
Because basically (in the worst case) you would then have 1 potion of each type and could only camp at fireplaces, making the "planning phase" very un-dynamic in such a big world