I would think that this is because those are what is currently available, and that this would change when actually capable humanoids start coming on to the market. It will become possible to re-use existing equipment (providing the largest uplift to farmers who haven't already begun the process of heavy automation, which is a lot of them when you look at a global scale). Humanoids will be more accessible than the bulking 8 ton autonomous "tractor".

Due to the scale of pre-training going on, it seems reasonable that a humanoid could also do a lot of the preliminary work you mentioned that currently is not (or rarely) automated.

On one hand I feel like I'm sure to catch some ridicule for saying any of this, on the other it seems like it is very obviously the direction we're headed.