Marathon is a victim of 4 things:
1.Sony’s live service reputation.
2. Bungie’s D2 baggage.
3. Online grifters who's most played game is steamdb player charts.
4. The playtests were terrible.
The game itself is genuinely one of the best shooters in a decade.
As brain-dead as the steam chart spammers are it really does make me sad that more people aren't playing Marathon. Underrated isn't even the right word, it's just artificially hated which is just a bummer.