a question

[ontheoutside]

todays 10th race purse is 50 thousand

they pay i assume 1 through 5

something like 50 % to 1st 25 % to 2nd 12% to 3rd 8 % to 4th
and 5% to 5th or something like that

so in tenth there is only 4 horses running

so why wouldnt one of the trainers stay in there for at least 2500 dollars for 5th/
thanks
im sure its a easy answer and ill be shaking my head when i read