I think there are a number of obvious answers:
(a) With experience, many can’t or have no way to distinguish the content and vehicle of experience: I don’t spell it out here but when I talk about the content of experience, I mean appearances, i.e., how things seem (with some possible extra “phenomenal=what-it-is-like-(but-not-how-it-appears)” that I must confess I’m not clear on).
This links to the diaphonousness (Crane 2010) or transparency of experience, the idea that all that’s apparent to us in experience are things in the external world, i.e., content. In that case, how does anyone distinguish this content from the vehicle?
We don’t have this problem with language: the sentence’s own properties are as obvious as their content. (Actually, the problem might be the other way. Once one goes beyond experience, one might have no idea how to pick out the content independent of the vehicle).
(b) Not everyone believes experience is representational the way language is.
That language represents the world, and does so with vehicles separate to content, is obvious — or at least seems to be. Perhaps a sentence is meaningless: there is no content for a particular sentence (‘mice bolster cannonballs’), but there is still the vehicle of the sentence, and that vehicle is separate to its content.
So it would be hard work to argue that a particular sentence is not a representation, even if were relatively easy work to argue that it represents nonsense.
But, perhaps because of (a) above, it is not (or does not seem) obvious that experience represents the world. And even if it were obvious that it represents the world, it does not seem obvious that it can do so in the absence of the content it represents. That is, it does not seem obvious that my experience can be of a tiger if there is no tiger. (Until one considers error, which gets in to thinking about experience in more depth).
And one final reason — not a defeater reason, perhaps, but one to consider:
(c) There’s a good reason to hold that vehicle and content can come apart in language but not in experience