Although SPACE wise I somewhat agree with you (although I won't go less than a 2.5 gallon personally) its not just the space that's the issue..
In a bowl you will not get enough oxygen exchange, the bowl will be CONSTANTLY cycling, and it will fluctuate in temp too much. All of which can be very stressful for fish. The only reason bettas "can" live in a bowl is because they have an extra organ that allows them to also exchange air on the top of the surface. Not ideal for a betta to have to rely on that all the time, so there will be some stress just in that. The bowl will not be able to grow enough bacteria and sustain it to keep the ammonia down enough to be safe for the fish. Bettas come from very hot areas, they prefer their water to be around 78-82 degrees and it to stay relatively consistent. In a bowl it stays about 72 unless the sun is hitting it.
Now there are ways to make a bowl work, I've seen it be done but its not just simply a bowl, fish, water, and decorations. Its people taking the step adding the heater, hooking up some form of arrerator, doing weekly water changes and keeping up with it.