They aren't relevant anymore for the same reason that USB cards, serial cards, network cards and SATA cards aren't relevant anymore. They still have their niche

They aren't relevant anymore for the same reason that USB cards, serial cards, network cards and SATA cards aren't relevant anymore. They still have their niche, and in commercial settings you'll still see HBAs (with SATA but mainly SAS), 10G+ network cards, and audio interfaces, but but for most of the personal computers the level of 'better than good enough' was already reached a while ago. There is no point to using an add-in card if the facility is now on the main board and can do the task to the user's wishes. The same thing can be said about many previously modular components where more and more is now simply a function of the main board itself. Take all the legacy I/O which used to be various chips, often on various add-in boards. They were all condensed into one single Super IO chip that can do all of it, but at a fraction of the size, cost and energy usage. A lot of peripherals used to be implemented in separate chips and sometimes even discrete logic. If we were to try to do that today we'd either have to cut 90% of the currently available standard features or make the mainboard ten times as big to be able to implement it the old school way.