I'm not sure about Kalshi, however on most sports betting sites you actually are betting against the house. The betting sites all have in-house models (or piggyback off other sites) that are much better at predicting odds than the general public. If someone is making money then the sites just place limits on that account so they're not losing money.

In a casino you can bet against other people too. Nonetheless, the house always wins and takes its cut.

There's an old phrase that says something like "Look around the table, if you cant see the mark you're it." - if you think you are not betting against the house on a gambling site, you're VERY it.