Because that sounds way overpowered. Any PC could gain damage immunities while still keeping their alignment. Being forced to transform into a bloodthirsty monster every full moon isn't that big of a deal if you find a way to restrain the character during that time. Even worse, some lycanthropes actually have their alignment shift to good if they embrace the curse. While this last bit can be solved by a savvy DM who knows that good lycanthropes don't willy-nilly spread their curse, it doesn't change the fact that evil lycanthropes don't care.

Rules: https://www.dndbeyond.com/sources/mm/monsters-l#Lycanthropes

Yeah, the curse has upsides as well as downsides.

Damage immunities are a pretty big upside, especially at low level, but the curse IS a curse. The characters should want to remove it.

One of my PCs was bitten by a lycanthrope once. I didn’t give him everything at once, and I only gave him the damage immunities when he wasn’t in human form. 

Quote from Tim >>

One of my PCs was bitten by a lycanthrope once. I didn’t give him everything at once, and I only gave him the damage immunities when he wasn’t in human form. 

 That's reasonable. I personally don't care for the way the curse works RAW.

This sort of thing is exactly why I'm remaking Lycanthropy for my own world (and to publish the rules if they work well enough). I dislike things like this which give players benefits without any real downsides/weaknesses. had a player who was a werebear barbarian and, whilst the game was fun, they were clearly much more powerful than the other characters.

Quote from ThorukDuckSlayer >>

This sort of thing is exactly why I'm remaking Lycanthropy for my own world (and to publish the rules if they work well enough).

If I had to remake the rules, I'd probably replace the damage immunities with vulnerabilities, at least for those resisting the curse but only in humanoid form. This wouldn't resolve the problem with good lycanthropes, but lore-wise they don't like spreading their curse to others. The only logical reason why a PC would resist the good aligned curse is if they're evil and part of an evil campaign, which is why I'd have no issue having the players face werebears willing to use their bite in such a situation. An evil PC in an evil campaign who shifts to good basically has to fall under the control of the DM. On the other hand, this kind of PC would have no logical reason to resist the evil version of the curse.

Another way to handle it is to rule that a PC unwilling to be cured of the curse at the first given opportunity basically means that they're now embracing it. Should a player complain, the DM can explain that despite the PC's best efforts, they became unable to resist it any longer. They were seduced by the power of the curse and thought wrong that they would be able to control it. It doesn't have to be a conscious choice.

I've broken the curse down into sections, based on how much they embrace it. I'm also making embracing the curse a real decision, rather than just a "do you want to roleplay a lycanthrope?". I'm looking forward to finally finishing the rules for it and having a go at playtesting it in a campaign!