It's so lucrative with such huge spreads, why don't more market makers compete in ETH? Isn't this guaranteed profit?
Let's face it. The business of market making is a JOB, and it's a job that requires intense focus and attention during market hours. When the bell rings and the markets close, most people in the business of trading (whether they are market makers, traders, or whoever) want to get the hell out out of the office and get back to their friends and families.
So when you have fewer participants in the after hours session, things slow down and the spreads widen due to less competition. This isn't necessarily a bad thing if you are on the right side of a trade. So if you bid up the price for a stock in the extended trading hours and your bid got hit, and then the company's earnings report or some other positive news came out, then yeah, you would likely see a major price increase because the number of available shares for people to buy in the ETH are few so the price jumps up fast and furious. As an individual trader, you could capitalize on that movement and actually have a chance of your sell order getting filled because of your limited order size, so all is good.
The problem of course is on the flip side. Let's say the exact same scenario happened: You bought a stock in the extended trading hours session thinking it was going to pop, but.... the opposite scenario actually played out, meaning that the earnings were shit and now the stock is tanking by the second. And because you are playing in the ETH session, the few bids that are actually present are getting hit by the second, leaving you with a 22% loss before you even realized what happened.
So in summary, trading in the ETH can be great if you're on the right side of the trade, but it's not a guaranteed money machine by any stretch of the imagination, and it's damn sure not your friend when you're on the wrong side of the trade.