I think anyone I know who considers marriage as “just a legal contract” isn’t saying that one should treat marriage as a contractual relationship like any other but rather that the ceremony itself is unimportant relative to the relationship as a whole. I think these are two very different things people may mean by the phrase and I feel like your comment suggests that one of those things is common while I would suggest that the other interpretation is more common.

If people thought of marriage as nothing but a legal contract, likely to be broken, and acted rationally then I expect they wouldn’t bother at all: the benefits aren’t particularly great while the risks are large.

I think we probably mostly agree with our own opinions but perhaps disagree on the opinions of others.