Because its easier to water/maintain and the root/plant growth goes quicker.
I find it way easier to just put the seed into the Coco, give it one good watering. People say it's easier and promotes faster growth, but every plant I have started in jiffys or cups never fills a whole 5 gal pot with roots. When I plant them straight into 5 gal pots I find they are rooting out the bottom in three weeks.
I'm probably useless watering, but I found it a complete ass to have to drips small amounts of water into a cup all the time to keep it moist. It just seems way easier to me to plant straight into pots, water once and not have to for a few days afterwards.
I must be retarded, but every time I plant in a cup Ur jiffy pot, I come back a few hours later and the tops dry lol. Moving back to dropping a seed straight into 3 gal pots for autos, and 5 gal pots for photos simply stopped my issue with over/underfed seedlings and gave them a steady environment for the first few days.
I've noticed too, that my Last few plants are 30-40cm tall in a few weeks, while most people are still in solo cups. I was in that same boat for a year or two until I got lazy and started throwing seeds straight in the ground outside, or in 3 gal pots indoors.