I mean, seriously, it's not "The One Ring", which came with its own set of problems for the benefit of making its wearer invisible. Invisibility is a 2nd lvl spell (Duration 1 min) although the duration is indefinite in ring form, you cannot attack or cast spells.
Compare this to a ring of Regeneration (Very Rare) which lets you regain between 144 and 864 hit points in a 24 hour period and will regrow your left arm if your last encounter went a little off the rails.
It seems to me that being invisible is much more useful in tiers 1-2.5, after which the enemies start to have abilities that overcome invisibility – true sight, blindsense, tremorsense, and so on.
Best Answer
Because it is game breaking
Having a Ring of Invisibility isn't just being able to cast Invisibility a dozen or so times a day, it's being able to turn invisible as an action whenever you want. A character with this ring can and likely will...
While not unassailable invisibility is a powerful, versatile, ability that requires particular abilities for monsters to handle effectively. Unlimited invisibility can make enough situations utterly trivial that an item that provides it is the stuff of legends.