"Why can't the call handler just say "it's ok, ambulance is on way, answer these questions in the meantime"?"
Because, as has been said many times, people then just hang up because they've done their "good deed" and can feel all virtuous without spending any more time on the matter. There was a poster up thread who did actually say "in the end I just had to say sorry about to miss my bus please send ambulance and hang up" - prioritising catching a bus over finishing the call to 999!
I've had to call 999 ambulances a number of times for various reasons, and I've never had any problems with how the call was handled. Reasons (among others) ranging from my own father (terminally ill with cancer) falling in the garden onto steps and bleeding heavily, through an elderly lady in the street who tripped & fell over a curb & hit her head very hard, to an unconscious man on a bench in the street. For my father, many years ago before the various health service issues, ambulance came within half an hour. Elderly lady, the call handlers questions allowed it to be determined that she wouldn't need an ambulance as her friends who came along said they would take her to the local cottage hospital for a check up if she wasn't improving in a couple of hours.
The guy in the street was actually in a diabetic coma and would have died without assistance within another hour or so. But I had community police there in 5 mins, a bike paramedic in 10 and an ambulance in 15. This was mid covid crisis as well, during the 2nd lock down.
Each time the call handlers were very much to the point, following a script that meant they couldn't miss asking important questions e.g. in all 3 cases whether the person had been drinking to my knowledge or if I could smell alcohol on them, what attempts had been made to rouse the unconscious man etc.
I've also been on the other side of the phone for non-critical calls but often from distressed people, and sometimes abusive. We do need to ask certain information like full name with correct spelling to find them on the system, then they can get angry we aren't giving them what they want right that second. We also get people stressed out and not listening to what I am saying, or misinterpreting it to fit their own agenda e.g. I'll say "I'll contact your allocated worker and she/he will be able to discuss this further" being interpreted as me saying "of course you can have that", or "no chance matey".
The common criticism from my boss is that when I'm on calls I tend to spend a bit too much time empathising etc rather than just getting the message to the right person, yet I had a complaint about me being rude and talking down to this caller because she only listened to half a sentence & then tuned out.