First, as @littleadv mentions, and as I've pointed out before, anyone who participates in a market using limit orders (which, by the way, should be every non-professional investor) is by definition a market maker. So, I will assume that your question pertains both to official market makers and to "retail investors" using limit orders.
When you remark that there are such "tight spreads" in "liquid assets", what you are really saying is "wow, look at all the market makers in these products!" That's the benefit of electronic trading and algorithmic traders -- millions of participants each with their own opinion of the value of a financial instrument, trying to find people who have very specifically opposing opinions of the value of that same instrument. This is called price discovery, and is the entire point of financial markets.
So, you ask why are there all these market makers present to create such tight spreads in assets like
SPY? Answer: Because they can make money in these markets:
Imagine (towards a contradiction) that market makers thought they couldn't make money by offering tight spreads in
SPY, and so
SPY had a wider spread than it actually does. For example, say the highest bid for
SPY was $99.98 and the lowest ask was $100.01. Now imagine that a market maker with perfect knowledge of the future came along knowing that he would be able to sell
SPY for $100.01 in 5 minutes. Then he would load up as many buy orders as he could for $100.00 or lower. (He wouldn't bid $100.01 or higher because those trades would not be profitable according to his information -- at least not 5 minutes from now.) So the spread had previously been $0.03 and then suddenly it was $0.01, all because a market maker with better information came along and realized he could make money by creating a tighter market!
Now, nobody has perfect knowledge of the future, which is why markets are never infinitely tight or infinitely liquid. Each market maker has to weigh possible profits against the probability that those profits will actually turn into losses. But if one market maker decides not to participate in a particular instrument, there's bound to be another market maker who will happily take his place. So the very fact that there are so many market participants with resting buy/sell orders for
SPY right now is proof that there are market makers able to make money doing so. If they could not make money, they wouldn't be there, and the spread would be wider.
10-15 years ago, before electronic trading and algorithmic trading, the number of market participants was far lower, and the spreads were far wider, meaning retail investors like you and me had a much harder time making money. The only people making money were the institutional investors, the brokers, and the exchanges. Now that all these new millions of players are present in the market, retail investors like you and me get to participate and make money too.