right, that's exactly why I was thinking
I was thinking that I was going to have to develop this idea myself before I even approached them
but why can I not just walk up to them and introduce my idea
(under the right circumstances, such as a signed NDA or something)
For the same reasons rappers can't just walk into a record company's building, get a meeting with the CEO or President, rap for them and then get a million dollar deal:
A company like Facebook is a billion dollar company and will not talk with people who approach them with ideas (as mentioned before) to avoid getting sued and losing millions, if not getting 'Gawkered' (sued, lose, go broke).
This is reason 1 why they pay attention to small/young companies who have a new product that is doing well.
The 2nd reason: they hired and hire really smart people who can come up with ideas and many of their ideas get executed and added to the product (e.g. when Facebook first started you could not add videos, share links, tag people in posts etc. but now you can).
The third reason: anybody can have an idea. But executing the idea and proving that there are people who want to use this product is something else.
This is how Instagram and Whatsapp and many more companies you never even heard of (because doing well can be 'we got 100.000 users in 2 months after launch' it is about potential too) got bought by Facebook. Facebook loved what they saw and approached the companies.
(I think it's great that you are interested in making an app
@D.C Young - don't let any comment stop you)