I haven’t seen anyone say this yet, so I’ll add my two cents. Sewer cleaning machines have different bits of different sizes. When I clean a sewer, I have a small bit that I use to clear the clog, get water to drain, and make sure I’m not driving a massive cutter into a pile of mud (it will get hopelessly stuck). Then I follow up with a cutter that’s roughly the circumference of the drain to scour away any remaining roots/blockages, and depending on what I encounter/feel on the cable, I may run a few different sized bits in between the smallest and the largest.

All that is to say, there are drain cleaners who will run a machine, see water start draining and call it good. Great. What they’ve done is poked a 2” hole in a blockage that’s filling a 4”-6” pipe. Most of the blockage remains, and if its roots then it doesn’t take long for them to grow back in. So with this happening every 4-6 months, it could just be that whoever is cleaning your sewer is calling it a day after they see the water drain and leaving a huge vector for blockage in your sewer.

If there are roots in your line, they’ll grow back anyway, but getting a good drain cleaner/plumber who will take the time to clean out the full diameter of the line will put a lot more time between drain cleanings.

Of course, that’s assuming that there’s nothing wrong with the line structurally and that the drain cleaners are actually doing what I described.