Yes, I'm aware of thin water atmo. landables. Let's put some numbers behind them, shall we? There are around 67,000 of them uploaded to EDDN, compared to around 370,000 ELWs. The rough chance of an ELW is around 3% per system in the best conditions (A/F main star, mass code D system), and the rough chance of a thin water atmo is around 0.4% per system in the best conditions (B main star, mass code E system).
So if you're looking for one that's undiscovered, that'll take a while. It's much quicker to just go through the crowdsourced data and go after what's already discovered, even if you won't be the first to log the flora. In the time it would take to find a new thin water atmo. planet, you can go through many more already-uploaded ones, and the first logged bonus is nowhere near enough to offset this.
Meanwhile, to reach Exobiology Elite, 432 million Cr is required (last time I checked), so that'll be grinding through nearly a hundred planets with the same plants and generally the same skies. Quite doable, sure, but it is a monotonous grind to rank up this way.