Most European and East Asian major cities do have that. A train every 2-5 minutes during busy hours. In smaller cities (below one million people or so), a train / bus / tram every 10 minutes is probably most common.

You need some combination of high-ridership and low-personnel-cost. You can get that in most of Europe and Asia, but incomes in North America are quite high and labour capture is high too. Most jurisdictions cannot afford the drivers necessary at high frequency because there isn’t enough density for it.

Skytrain bypasses this by not having drivers and perhaps as buses become AVs we could add many more of them here in the Bay Area with much higher frequency.

Most big European cities run trains every 10-15 minutes and every 5 minutes during rush hour.

This exists in many cities. See Bus Rapid Transit and many rail lines. Seattle's network has many instances of this.

If I were to speculate, I'd say that perhaps it might only be feasible in small & dense cities with low traffic, as such high frequency requires a lot of resources.

Was born in a small city with frequent public transportation (5-10 minutes for most routes). As I moved to larger & more congested cities, public transport became less convenient and the schedule more erratic.

In Sydney we are very close to Asia so the shame of our rotting train system finally caused the city to invest in modern driverless underground transit, termed 'the metro'. I believe planning began in the 1990s and it only really launched about 5 years ago, so 20 years or so in the making. This system is much faster, cleaner and more modern than the older surface rail, largely helped by the fact it's way underground in most sections due to a lack of surface land availability. However, it frequently stops or requires maintenance and is run by a totally separate bureaucracy than the rail system, who apparently refuse to communicate despite having shared stations. This leads to situations such as one encountered this weekend whereby the rail operator had track work scheduled on a major section of their route, and simultaneously the metro system had disrupted service, such that a great number of passengers were stranded. More importantly, even when it is working they do not coordinate arrival times, such that it is exceptionally common to arrive on one mode, where the platform faces directly the other mode, but it has just departed. 30 seconds of scheduling either side would greatly increase efficiency, but it's nobody's department. This is the sort of thing they do perfectly in Japan, but Australia is a country of 1000 little scams with everyone fervently minding their own scam. We urgently need to replace corrupt humans with AI.

> If only most cities could have this today

Judging by the rest of the replies (most of which I'd agree with), it seems more uncommon that they do not, than they do :) I think this heavily depends on how friendly your government in general is to public transport, which can vary heavily if you're in some specific places in the world.