Not "on" but "under"
1) Life on Earth appears to have evolved in a subsurface chemically-pumped environment
2) Early Mars had similar environments and "might" still have them today
3) Life on Mars today is only likely in the deep subsurface where fluids exist
4) The search for life on Mars can be proxied by a search for fluids
5) We ain't found any yet, but we haven't probed deep enough...