As the above posters have said there is more money in the sport at the higher level so they don't need to fight as often.

The other main reasons are health and performance.

With the amount of money at stake at the world championship level no boxer can afford any off nights so each fight must be meticulously trained and prepared for.

Finally with all the health risks associated with boxing there are probably few boxers, and even fewer sanctioning bodies and medical boards who want to see any modern day boxer fight hundreds of times in a career with a different fight every month.