Because looking for life on mars sells.
"Why are we not spending that money on more social housing?" : Because this is the greatest mystery of our time... (nods all round)
"Why are we doing science?": I don't know and I don't care, cut my taxes! (Cheering and mocking comments about nerds.)
This is where we are at on HN.
Conspiracies that NASA already found alien life and is suppressing it to continue funding.
Or you know the far more likely case:
No one has ever found any evidence of life anywhere and exaggerating tiny findings adjacent to it keep funding flowing.
No it couldn’t be that.
If life were found on Mars then the greatest mystery of our time would be whether life originated at least twice independently, or if we share a common origin. Both of which have significant implications, to put it mildly.