If you care about achieving a particular mission, you could view joining a large funded organization as an opportunity to achieve that mission with higher likelihood or speed.
For example, if you had an idea about planting trees via drones and a big company said "come to our company and we can get this launched at scale as early as 2026". Then you think about how long it would take to do it on your own and you'd still have to convince investors and hire people and stuff.
If at the end of the day, what you care about is planting more trees quickly, you should probably join that big company. (of course there are risks too, where they cancel the project or divert its direction).
And also, not every startup found cares about achieving some grandiose mission. Some just want to get rich which is totally fine.