Because hearsay is an out-of-court statement, but there are exceptions, including a statement against the person's interest. FRE 804(b)(3).*
In other words, if you try to introduce an out-of-court statement that supports your case, it's hearsay; if the statement is against your interest, it's allowable.
Of course there are other exceptions and nuances, but this is the jist of it.
But it’s not like that acts as a filter against anything in your interest, since it would still come out on cross, right? I tried to illustrate with my comment here:
In other words, if you try to introduce an out-of-court statement that supports your case, it's hearsay; if the statement is against your interest, it's allowable.
Of course there are other exceptions and nuances, but this is the jist of it.
* https://www.rulesofevidence.org/article-viii/rule-804/