What services or apps did you see abroad and wonder: why don't we have them?

What an Orwellian nightmare. These systems require real-time GPS data.

Just as buying a ticket with cash is becoming increasingly hard in parts of Europe, I can see a near future where having a phone sending constant GPS updates becomes a requirement (a requirement in an strict sense, or the sense that the alternative is unreasonably cumbersome or more expensive)

I believe Copenhagen has this too now. I visited in the same week they were launching the system (last year?) and it worked perfectly. All you had to do was press a button on your phone as you enter and leave the station and it automatically calculates what you owe. I loved it. It also worked throughout all of the different train systems.

More than that (which is an amazing feature, don't get me wrong) is the fact that there is a single app for every public transportation system in the country.

Compare that to Italy/France/Spain (those that I know) where, depending where you are traveling to, you have to download, sign in, and give your credit card details to N different apps in different states of disrepair/being barely maintained.

Virtual credit cards (I use Revolut) that I then delete mitigate that, but still, what a mess.

Certain parts of France and Germany, that I know, use something similar for regional trains and buses: FAIRTIQ

Queensland, Australia introduced state-wide $0.50 public transit fares a year ago, and it’s been a raging success. Conveniently, this also eliminates the entire problem class of needing to calculate fares. Mind you, for those unfamiliar, QLD is a state 2.5x larger than Texas, 5x larger than all of Japan, 7x larger than Great Britain, and is bigger than all but 16 countries.

This. It also caps your fares in a day to the cost of a daily ticket, so you don't get overcharged.

Sydney too, for quite a while (I am sure at least 5 yrs maybe 10)

In London contactless payment via credit/debit/Apple/Android has automatic daily and weekly caps

https://tfl.gov.uk/fares/find-fares/capping

Depending on the frequency of travel, it can be cheaper to get season tickets though

I don't think so (not a Londoner)

It was my impression that the whole thing was just about simplification in order to provide a better service.

There’s probably an economy around those keys - people who lose them have to pay an (overinflated) deposit, some company is overcharging the locker owner for key replacements, etc. At every layer someone skims a bit of money, so nobody in power of changing the system is actually incentivized to do so.