I don't think it's so much a matter of them not investigating as it is the manner they go about doing it is extremely flawed, because people wouldn't be getting banned at all otherwise (I've read first-hand accounts from botters who got banned that show that their numbers aren't just empty fluff).
I suspect they are not doing actual in-person investigations and are instead just using a packet viewer or some other monitoring tool to look for unusual data being sent from a player's client as the botting programs do allow people to do things that are not normally possible using the in-game control schemes. However, even that seems flawed as I've witnessed things like bots triggering skills while they're still in the middle of cutscenes or toggling gathering abilities while they're still on a loading screen (This manifests as buff notifications appearing in an empty space next to the aetheryte they're warping some five to 10 seconds before they actually load in visibly). I've also witnessed them targeting things through walls as well as keeping things targeted from far outside the range a normal player would be able to (Mostly can be observed with aetherytes).
The RMT bots can't be controlled with bans. They can create new accounts literally the instant they get banned then hit level 50 again within 24 hours, leaving them with the rest of the week to farm gil; not to mention the bulk of their gil is obtained via quest completion anyways so they've already amassed a sizable sum by the time they start farming Amdapor Keep.
Also, they almost never actually ban buyers and the accounts doing the selling are expendable. The vast majority of the bans you see in their reports are for their expendable farm bots, with a fair amount of bans also being issued to their spam bots, but ultimately, buying gil being so low risk is partly why the trade is able to flourish.
On the other hand, it takes a substantial amount of time for people to create even just a DoL bot due to the MSQ locks and what not, as they need to complete all of the MSQ quests manually even if they opt to get around the EXP walls by FATE botting.. They could easily be kept in check if their enforcement teams were capable of identifying their repetitive behaviors and abnormally long periods of activity.
No, having the bots gathering with other players isn't harmful due to the thankfully instanced nature of nodes, but having them selling things at prices that are scarcely profitable for people who are actually playing the game is. Also, there very much are DoH bots who are tied to the RMT trade. There's one on Cactuar right now who's so persistent about trying to control the market via 24/7 gathering, crafting, and undercutting that I can't imagine them doing it for any other reason and they're very obviously an alt account judging from how the absolute bare minimum has been put into everything about them that isn't tied to DoL/H classes.