Sending a microscope is easier said than done. Many natural structures can look like bacteria, and vice versa. If there's more complex single celled life then we might see stuff swimming around but that's considered unlikely close to the surface where there's a decent amount of solar and cosmic radiation. If complex life does exist it's probably deep underground or in caves and lava tubes where we can't reach it yet.

The other reason is planetary protection. The best places to send a microscope are low lying areas where there may be brines near the surface. Those specific areas have been designated high on the list of protection sites. Earth microbes are really resilient, so even with intense sterilization procedures we can't be 100% sure. We don't want to contaminate the most valuable scientific find ever, and so we're approaching it carefully.

But I think the first reason I gave is the most significant one. It's technically pretty hard and not definitive. The surface of Mars is probably mostly sterile even if there is life. If it survives, it's probably underground.

I also disagree that NASA would not want to find life. If anything, finding life would make their budget explode. They could suddenly make a strong case for a Europa submersible, a submarine to visit Titan's methane lakes, huge space-based SETI radio telescope arrays, huge space telescopes to try to find more exoplanets and characterize their atmospheres, all kinds of things, since we'd know for a fact there's life out there.

If life emerged in two places in one solar system, we'd know that the universe is teeming with it. Maybe not complex intelligent life -- there's still reasons to think Earth may be kind of special for that. But life, certainly.