Windows font scaling is the reason.
I think money and practicality are the reasons.
4k (for the most part) started with high-margin 55''+ tvs, where the practical use was not only arguably the most tangible, but are/(were) also more lucrative (for the panel maker) than a smaller monitor, not to mention it made it more exclusive (which keeps the margin higher longer). Obviously a monitor vendor wants the opposite; they can make their 30% markup per higher-selling commodity item, but they are reliant on the panel makers and specs to make that feasible. Also, just like a phone/tablet, a tv is going to likely be a more passive experience with less stringent guidelines (be that color, refresh rate, response time, input lag, input options, etc) and that's where (until recently) we sat.
Monitors are getting there now, as <70'' panel prices (across resolutions) are expected to take a substantial nosedive by the end of this year (If you've noticed a lot of September release announcements...that's why), and as (like I've mentioned before) the panel suppliers are beginning to get their panels at high-enough yield rates and specs (along with inputs like hdmi 2.0) to make them feasible to cut down and sell in bulk for monitors (obviously at least up to 32'' from auo). The question then becomes one of practicality/market for something like gaming...and I would argue on the whole the ecosystem is not there yet (meaning for the average consumer). No doubt this year and into the next it will drop the level from 'elite' to high-end, but things like true adobe RGB color/new displayport are things that will probably help a lot of people make the jump when conditions are more ideal. As such, that's likely when they will get the most attention by suppliers. That's not to say there won't be landmark products along the way.
As for the AA question, I agree it's a case-by-case, but personally, I use a metric of 20/15 vision as many (most?) people fall roughly into that criteria. Arguably it doesn't matter if you go by 20/15 or 20/20 though as the result is similar; in many cases 4k will benefit from 2x AA (or essentially 8k), but it certainly is less predominantly required for many people's scenarios than before 4k.
To make it simple using 20/20 vision:
28'' at 2' would be the threshold for perhaps typically not really needing it. Might you sit closer or have a relatively larger screen/distance? Sure. Might your vision be slightly better? Probably. That is why I think 2xAA will remain a thing (for those using up to ~40-42''/2ft, ~32/20'' etc)...just like 8xAA was preferred, but 4xAA was the sweet spot for 1080p.
*sigh*
