Not sure on SPY but SOXL today had bids about 10-15 cents higher than asks for the first 15 min. Candles kept jumping around. I literally bought at ask for 21.43 and sold at 21.52 bid in one second.
True but if you are ready and patient (not me) then you can make bank.
It happens a couple times per year.
Besides, there are tons of leveraged etfs out there.LABU LABD KOLD BOIL SOXS SOXL NUGT DUST only to name a few.