There are a few issues to take into account.
1. The software itself is doing far more than on the N64 or PS1. AI, visuals, sound, scope & scale plus the near-omnipresent nature of online all contribute to making writing code that works perfectly a lot harder.
2. Online & networking adds a whole new universe of complexity and problems.
3. Online also brings with it the means to easily distribute fixes for faulty software. Couple that with the low cost of physical media or minimal cost of digital distribution and you don't have anything like the monetary risk associated with manufacturing millions of cartridges.
4. It never ceases to blow my mind that seemingly so few people on this board work with business software, or at least experience no problems with it. Real-life business issues like security flaws in OSs, Java, browsers and so on make the concerns expressed by gamers about whether multiplayer seem pretty small TBH.
Outside of actual mission-critical process control software (i.e. the stuff that runs powerstations, factories, stops planes falling out of the sky) lots of software is buggy and delivered in a half-ready state, even years after launching (yes, I'm pointing my finger at Windows on that one).