NASA's Perseverance Mars Rover

Shortest answer: quartz has to be separated from other rocks/minerals. Water action is one of the easiest ways to manage that. In addition, opal/chalcedony is actually quartz with water directly attached on a molecular level, so that's a direct discovery.

Medium answer: Igneous ("volcanic") rock already contains the silicon and oxygen that quartz is made of, but they're usually bonded with other elements, not just each other. In other words, they don't exist as "free quartz" - meaning independent grains that are made of pure SiO2. As @athairmor alluded to, free quartz can form directly from magma when it solidifies and forms igneous rock. However, that is what you would expect from particular kinds of volcanic rock, which are absent or rare on Mars (e.g. granite). The igneous rock around Jezero Crater is not the type to contain "free quartz". If the regional geology hasn't served up any free quartz grains directly, you can still separate out the silicon and oxygen by breaking down the larger, more complicated minerals they're attached to, but that would take a significant amount of chemical breakdown - i.e. significant amounts of water. This process is quite common on Earth, of course, where it yields up "white sand" on beaches - which is simply rounded grains of quartz.

Longest reply: I should probably just read the EPSL paper, and I'd be happy to summarize it here if people are interested.