My theory is that it's better to err on the side off caution. If the weather is ready before your seedlings are ready, all you lost was a few weeks, but the plants will go straight from your lights into optimum conditions.
If your seedlings are ready to transplant out before the weather is ready, you'll have root-bound, leggy, unhappy seedlings to plant out into weather barely warm enough to keep them growing.
Often a late transplant will catch up to and even pass an early transplant - especially if the early transplant caught even one late cold snap.
And for tomatoes, a night-time temperature much below 50 F could count as a "cold snap". I guess some people plant tomatoes out before nights stay above 50F, but I think they get sulky and slow their growth if they get many cool nights like that.
The "last frost date" isn't all that matters to some crops.
