I agree it can be inconvenient. I think the objective is to give teachers time to prepare for their classes, but yes, sometimes I'd like the option to let students book at the last minute, when we both have time and agree to meet! The way I work around it is, we have the lesson anyway, and they pay by booking a lesson for the next day (which we don't do). This only works if the teacher and student know and trust each other, of course.
A note about teachers' schedule: "time available" doesn't mean I'll be sitting at my computer the whole time! It simply means time available for booking, preferably in advance. Depending on which times are booked, I'll then arrange the rest of my day around those lessons. That's how I can say I'm available 11 hours a day. It's to accommodate students' schedules! :)