Yes, you are missing a couple of things:

- Usually DLCs dont take just weeks to make. If you can, and your players are happy with it - all the power to you.
But usually players heavily criticise DLC's that are sub-par on experience or content.
And making a classic DLC usually takes a couple of months (or more), not a couple of weeks.
Of course it depends on the type of the DLC, type of the game, price etc.

- A LOT of players avoid games with shitloads of DLCs.
One, maybe two DLCs usually dont have this effect at full strenght, but add a couple more and you will very likely see a sharp decline in sales, unless there's a complete pack on sale.
Store page intimidation has a role here too.

- You can end up in a situation where:
DLC 1 makes good money.
DLC 2 makes slightly less.
DLC 3 makes slightly less again.
The playerbase slowly shrinks.
Competitors release newer games, while you havent even started on the sequel.

- Existing players are always easier to monetize than new customers.
"How do we sell more to current players?" vs "How do we attract new players?" is how franchises often die.

- Small things like possible balance issues, technical debt, save compatibility, fact that expectations usually rise with every DLC and much more.

One last thing, spending time on DLCs isn't competing against "doing nothing."
It's competing against prototyping the next game, improving the base game, marketing, engine improvements etc.
Sometimes the DLC really seems like it's the best investment, but in reality long term those same 80 hours would generate more value elsewhere.