The reason horses get into odds-on is BECAUSE there is no other horse near them in price. If there was there is usually not an odds-on runner.

Odds on runners lose between 40-50% of their races. You need a price of $2 just to break even in the long term which of course your not getting at odds-on. You might just be having a lucky run but long term there is no future backing short prices just because they are short and seem to have no competition.