Typically there aren't two separate tanks - In one tank there are two tubes at different heights. As the fuel level falls below the height of the "main" tube the engine sputters, then turning the petcock engages the lower down "reserve" tube which is still below the fuel level. It's more of a warning than a true reserve, and most bikes with an actual fuel gauge don't have a reserve.
On bikes like that, there's a reserve-reserve trick sometimes. Sometimes, the tank is an inverted U shape so when the pickup runs dry there's still a little more fuel on the other side of the U. If the bike is light enough, you can stop the bike and lean it way over to pour that last bit over to the pickup side. Might get you another couple miles.
Fuel injected motorcycles don’t have reserve (at least, none that I’ve seen.) instead they have low fuel lights or full fuel gauges. I’m guessing it’s because the fuel pumps are in the tank and the fuel injection system needs high pressure.
Fuel injectors require filtered gas because even small particles can clog them, and said filter is more likely to be clogged or even compromised by sucking up the last drops of fuel (and scale and debris) in the tank, so the low-fuel warning is required.
Carb jets can get clogged, too, but are wider since they're not under as much pressure. Also, since they're a wear item they're a lot easier to clean and/or replace.
I think grandparent commenter had it right: it's because the pump is in the tank. There's just no good way to have an external petcock determine where a tank-internal pump gets its fuel from.
Many new bikes come with a lot of rider aids for safety (ABS, TCS) as well as all kinds of electronics (fuel maps), so this is changing. But of course manual transmission won't go away until bike are electric.
I am one of those who likes things old school. My bike still has a carburetor, has no fuel light or tachometer, and I have certainly had some practice reaching down to turn the fuel petcock to reserve while sputtering on the highway. If they didn't intend for me to do that, why did they put it on the left side? :)
Goldwings also have a reverse gear.
Even more remarkable: I used to have an Aprilia scooter that had a remote release button (on the key fob) for the under-seat storage area. I think I used it once just to see if it works.
Some newer bikes, like mine, don't have a reserve petcock. They have a low fuel light. No forgetting about the petcock and an obvious warning light instead of sputtering.
Some older bikes, like my '99 Ducati Monster, don't have a petcock. It has a low fuel light that first failed in around 2002, and for which that part that fails (the in-tank float switch) stopped being available in about 2015 or so. No petcock _or_ warning light. (And that trip where the speedo cable fails so I couldn't even use thew trip meter to estimate fuel requirements was a fun one...)
Can you find someone who can adapt a float switch from a different bike? It seems like a very useful thing to have, even if it's not the original factory part.
I've just gotten used to it. I'm fairly reliable about always resetting the trip meter when I fill it up (and always fill it to full). I know it'll get 200km easy, maybe only 180 if I'm having _way_ too much fun. That's always about time I want to stop and stretch my legs anyway. It doesn't bother me enough to "solve the problem".
Most motorcycles with a manual petcock are very manual in nature. Often this is to minimize the number of moving parts that could die on you if you take it into rural areas. An automatic petcock adds more complexity that could cause a malfunction.
