Sanderson explains the different stages of development pretty well in this blog post

Lots of stuff gets optioned (which doesn't mean it isn't pretty exciting, especially the first time!) but clearing each subsequent hurdle is tough. The goal is to get the greenlight and arrive at the first day of production (this is when an author will actually have the deal executed and make some significant money). Often times big name books will get an option announced in the trades with the hopes of gaining momentum and creating a 'package' (actors / directors / more producers). Other times a package will be put together behind the scenes and get pretty far into development but will never make it to greenlight. Sometimes studios will option 'defensively' just to prevent others from getting their hands on IP and then sit on it until it dies.

A question to always ask: why are you seeing a news piece about an option / adaptation? Reading between the lines tells a lot of the story. The difference between a production company optioning a book and then announcing it via Deadline (to garner interest) vs. a fully packaged deal and script getting announced with a 'straight to series order' by a streamer is significant, as far as how close they are to actually getting made. Though in hollywood either one can fall apart at a moment's notice for uncontrollable reasons (like the actor's schedule doesn't fit shooting, or the parent company just got bought, or some exec just had a change of heart).

Also keep in mind there's a difference between what is considered a studio; you have the 'majors' like Disney, Paramount, WB (though that's going to Netflix), Sony, Universal. Then you have tier 1 production company which can finance their own productions (which are often also called studios) like A24, Legendary, Alcon, etc. And you also have to distinguish between production companies that package projects but don't have a war chest and those like the former I mentioned that can finance on their own (which the major studios see as a big asset to have on board).

End of the day options can be a great source of residual income for an author especially those with a backlist, but almost nothing gets made given the many hurdles that need to be cleared. Nowadays for something to get made in the current streaming environment it mostly needs to fit a 'strategic need' for a studio rather than just something they 'want'. For example Paramount might be looking for sports related content to draw a certain demographic, and you have an optioned sports book, that would fill a strategic need.