If people wanted to vote for a third party, they would. Money being spent can’t prevent them doing that.
People don’t vote third party because it is basically wasting a vote. That’s a consequence of the system, that needs change to come from within, and people in power won’t vote to reduce that power.
It's also substantially due to both major parties being pretty big tents; they're effectively coalition groups in a trench coat, in a lot of ways. Third parties that actually do stuff people want... end up caucusing with, or joining outright, the two big ones, because they're already pushing for some or all of what that smaller group wants, and it's the best way to get the weight to maybe actually do something.
The ones that
don't do that, are largely so incompatible with what people want they end up being a non-thing so far as political impact goes. It's a pretty consistent pattern in the US.
... that said, I'm not entirely sure there
is a lot of support for a third option, unless by third option you mean "stop friggin' bothering me". A great deal of the voting population largely just seems to not want to engage with the political process at all. Good bit of that's structural due to difficulties in engaging (fuck you, GOP), but not
all of it.
Not even sure how much I can blame them, exactly. It'd be nice if things just... worked, and you didn't have to worry about it. Shame the state of things hasn't been like that since at
least the southern strategy, more probably since before the bloody civil war