Why not sell even longer dated options, so you can get even more premium?

There are several reasons:

1. With longer-dated options, you’re tying up capital for longer periods of time, without necessarily getting proportionately higher amounts of premium. This reduces your ANNUALIZED return on capital. 
2. Since time decay increases exponentially, when you sell a longer-dated option, time decay is working in your favor at a much slower pace. Time decay relay starts to speed up at the 60-day mark, so selling options at 30-60 days is the sweet spot.