There will always be a "shortage" Because a game this big has a lot of moving parts. You cant work on everything at once. And just hiring more people doesnt help because hiring someone takes a lot of time and energy and the more people you have on a team the slower things happen.
When they were a small indie company they could imagine a champs kit hack it together in a few weeks make a short hype video and release it.
Now, they need to have tons and tons of meetings just to decide which faction in runeterra a new champ comes from.
Same thing with art. They have meetings and meetings to decide where they should allocate artist resources.
Yeah more people means more man hours are spent on the game but the effectiveness of each person is much lower in a company of 500 employees vs a company of 15 employees. This is why Riot always takes a conservative approach when hiring Designers and Developers.
(Working in a startup that was 11 employees when I started and at 200 employees now I definitely understand the struggle)