I'm not sure what kind of flippers you are referring, but usually there are no loans. Cash purchases - 100% for the entire value of the property.
Source: my parents are prof. flippers.
This is pretty unusual for anyone doing it seriously. I've "flipped" (not exactly -- mostly major additions and teardowns that we hold for 6-12 months) the past 5 years and haven't met anyone who exclusively uses their own cash. We use some of our own cash, but you have to look at the opportunity cost.
The interesting part has been the interest rates. We started out borrowing hard money at 14% and are down to 7-8% now depending on the lender. Lots of cash out there chasing returns.
No disrespect to your parents but if you leverage you can flip more homes. Higher risk, higher return. If you have the credit rating for it, interest only loans are how folks do that around the Bay area. I personally stay way away from that sort of risk.
Aggressive flippers. If you have enough cash to fund N home flips, someone will lend you money and you can do N + X flips in the same time (if all goes well....)
How many properties at a time can you flip with just cash? You also need to consider the cost of renovating the property, even professional builders need financing for those kinds of things. I guess you could do it out of pocket if the scale was small enough.
The interesting part has been the interest rates. We started out borrowing hard money at 14% and are down to 7-8% now depending on the lender. Lots of cash out there chasing returns.