Por que meu caldo de legumes é amargo, mas o caldo de galinha não?

I notice that your recipe doesn't include any salt. That's important, because salt decreases the sensation of bitterness. Chicken contains a certain amount of salt, and I suspect that's making the difference. (The "umami" -- brothy -- taste of chicken may also decrease the sensation of bitterness, though as I understand it there's still some disagreement about that.)

Try mixing 1/8 tsp of salt into one cup of your vegetable stock as a test. I suspect that'll decrease the bitterness to a comparable level.

Oh, and if you want to make your stock less bitter without making it more salty, use parsley stems and leaves instead of parsley root, and celery (including leaves) instead of celeriac. Those two roots will be the primary sources of bitterness.

Incidentally, I very much approve of you não salting your original stock, and instead salting whatever you use it in. Unsalted stock is more flexible, and is more forgiving if you decide you need to concentrate it.

I make veg stock overnight in a slow cooker on high with similar ingredients to you: onion, garlic, carrot, bay, peppercorns. But: celery instead of celeriac (I grow celery and often have some old tough stems and leaves which are perfect for stock), rarely parsnip or leek, and often some other herbs or veg I've got to hand. I don't add salt, and my quantities are a bit random, but I don't have problems with bitterness. The slow cooker maintains a very gentle simmer.

I also don't brown the ingredients first, but the bits that stick out start to caramelise by the end. I've never had trouble with bitterness, and wonder if your garlic, onion or leek may be catching a little, if you fry them first or if they end up stuck to the bottom of the pan.

It's unclear which vegetables, herbs and spices are being used and how they are being prepared; eg. onion can easily turn bitter when hacking them (which is squeezing them), instead of cutting them with a sharp blade. Sorrel, lovage, bay leaves, chervil, marjoram, rosemary, tarragon and fenugreek as well as cardamom, ginger, pepper, paprika, agrimony and thyme tend to introduce a bitter taste. And it could also be a timing issue, when adding certain herbs too soon. If unsure about it, just leave out an individual ingredient on each attempt.

I'd first try to leave the bay leaves out - or add them only towards the half-time or even later. The point is, that bay leaves harmonize nicely with meat, eg. when cooking Jus I cook them for 5-6 hours - but without that meat taste (umami), their taste is simply too dominant over the other ingredients.

If eaten whole, bay leaves (Laurus nobilis) are pungent and have a sharp, bitter taste.

Fonte: Wikipedia.