Because if you will wait for more dip, it might not happen anymore. We are about to get and enter into the bull run.
For that reason, buy if you can still cheap altcoins that you are planning to have because if you do not and you see them skyrocketing, who would you blame?
Investments in altcoins have always been risky and it always will be, so one is required to do their due diligence before they make an entry in any altcoin because you can't know how good or bad it is just by looking at it in the market and how its price is going up or down.
Having a certain amount of liquid assets ready like fiat money is actually part of a solid money management strategy. In bull runs I always try to build up 10-20% of my total assets value in fiat money. If we have a correction I invest those liquid assets into the market, so buying cheap is the idea. I usually do not do this in one buy, but in several smaller buys, due to you can not really know how strong the correction will be. As soon as the market enters a bull run again, I build up fiat assets again for next correction.