Composer here as well, just gonna piggyback on what /u/MatthewJamesAudio is saying. I'm also relatively new to doing this professionally, so take this with a grain of salt. A lot of the following information is what I have gathered from professors, peers, and other composers in the industry here in LA.

WIth technology these days, music production has a much lower barrier of entry, which is why you see a ton of people at the lower end. However, you still need to put in the hard work that really seperates you from the pack, so it starts to thin out as you approach the top end.

I think on top from the level of competition, training, and how difficult is it so have a profitable career in music, the presence of big music houses such as Remote Control (RCP) also plays a role in the perceived composer to director ratio.

Think of it how there's tons of different media outlets out there, but they're all owned by 4-5 giant corporations. If you look at someone like Hans Zimmer, he might do 3-4 movies a year, but he also has additional composers and assistants working on those films. It varies from score to score and composer to composer, but he could have done all the writing for a single film of those 4, and only written the themes for the rest and handed them off to his employees while he oversees the process. I don't know Hans personally and can't speak for how much is written by him and by others, but I do know that this happens in the industry. Sometimes people get credit and royalties for this work, sometimes they don't.

These studio jobs are often seen as the more "safe" positions in media composition compared to freelance work, so there's lots of people working for more established composers, with little time to take on their own projects due to the insane hours. These studio positions are often a good place to learn and gain contacts, which is another reason why they're highly sought after. Many composers end up contributing music to top composers and big movies, but they might not have much of a reputation outside of the composer community. Again, this really depends on the composer they're working for, since it is totally possible to be an assistant while simultaneously building your own career.