Lots of reasons!
You can't take most powered chairs/scooters large enough for me through the airport. Especially if you are heading internationally. Something something batteries, folding.
If you want to get a powered chair that I could use, it costs about $35k USD, and takes something like 15 minutes to fold. Also, lots of reports of Airline baggage handlers in the US trashing them.
Powered chairs and taxis don't work very well at all. US ADA regs for very, very narrow chairs; too narrow for me to use. Thus the "wheelchair cabs" (on the exceptionally rare occasion a city has any) don't suit. Manual chairs can be folder and shoved in the trunk of any old towncar, or into the back of a van.
The majority of vendor parties (or for that matter, hotels!) that I am expected to go to won't support either form of chair. They especially won't have doors wide enough to support non-folding power chairs. If you have to leave something abandoned at the front desk and hope noone steals it, which would you rather a $35k power chair or a $2.5K manual chair?
Either form of chair adds a crazy amount of hassle to airports, cabs, hotels, events, you-name-it. Now, that said, I have tried the manual chair thing. There is a problem with that too: I don't need one for my regular life. This means that I am nowhere near fast enough or powerful enough pushing that thing myself to keep up with people. (Also part of the reason is my arms are too short for most manual chairs; the wheels don't come high enough and i have to contort weirdly, which reduces power.)
For some events you could hire scooters/chairs (powered or unpowered) at the event, but not at all events. Even then, these may or may not meet size/weight/whatever requirements of the passenger, and sometimes are only rentable for the duration, not just for the event floor. (Leading back to the issues above.)
So it's not that events are undoable, it's that the logistics really, really, really suck.