In these conditions I even build something, as soon as the camp is finished I attack. i usually use fians and have good medicine, my brutality has no consequences for my troops.In general, I usually besiege cities with 100 militia and 100 (or so) soldiers, and the result tends to be that with at least 500 troops victory is assured with minimal (single digit) casualties if you simply do the Trebuchet trick (I wouldn't want them to be even more effective than they already are--the other weapons need more desirability, or durability specifically, since I've had no real results with them I couldn't have gotten better and faster with Trebuchets). I think it'd be much better if sieges were much harsher for the attacker, so that you'd ideally outnumber them 6-1 to get the kind of easy steamrolling results I'm describing. 100 total troops ought to be roughly equal to 500 and armies ought to regularly be huge 1000+ stacks to topple your average city/castle. I think as long as it remains cost-prohibitive to have more than 1-200 soldiers in a city/castle, this numeric balance could work.
And unfortunately at least in my game, that's the most common type of siege I have, the one that's not worth building anything, just charging at them. Only in quite late games, in cities super isolated from the war, usually in Aserai or Vlandia, can you get the challenge of using machinery first.
Without a doubt, it would be better, as you say, that the sieges demand a lot. They should consider improving the militia, in equipment and skill, as well as having a strong initial recovery, or the wounded militia cannot be captured, return to the militia. That the garrison improves in experience more strongly, that a city has 200 soldiers, but cat 5, some impact. that the AI capture and take care of the newly conquered and avoid the daily change of hands, that the nobles stop looting villages that do not need to be looted, mainly the city that they are already besieging (it will be yours shortly, stop doing that!) Everything takes a coordinated job of improvement that they should do.