So, I have seen several GM say that, and say that they allow it. But then, they turn around and make all plots time sensitive, so that if the players stop and take weeks to go off on quests for recipes, and more quests for ingredients, and then take even more time in crafting, well, the BBEG stomps all over the place, killing people and making advancements far beyond the making of a single magic item the PCs can achieve in the same time.
So, to folks who say that, you actually have to put your money where your mouth is, and not just nominally allow it, but not actually penalize the players for doing it.
For 'build a bear' systems, I like the 4e D&D crafting system where you can basically make anything in an hour.
Long crafting times work well in sandbox games. If it's a time-limited quest game where crafting takes a long time, the PCs should have a reasonable opportunity to be aware of the downside and plan accordingly.
Running Red Hand of Doom in 5e, I like its approach of simply throwing a ton of magic items at the PCs via all the NPC enemies - it is both flavourful ("These guys are serious!") and practical. But I also like the standard 5e official adventure approach of making magic items fairly rare & not necessary for success. It gives a different feel from 3e/4e, a bit more grounded.