because any attempt at concentration could be quickly observed and brought under artillery fire.
I don't think concentration does what it used to do in ww2. There has been a revolution in FCS that could have forced a state transition between musket blocks and rifle fire, but for AFVs.
With ww2 FCS where accuracy is low and multiple shots is needed for a hit, pen and kill, a numerical superiority means Lanchester R^2 law applies and the numerical superior side wins. Now in land warfare there is terrain and so on so the actual effect is not as strong as in the naval domain, however it still ought provide a significant advantage.
With modern systems with high probability of kill, numerical superiority does not result in improved exchange rates much, and is all negative when taking into account of artillery and logistics. Defender's advantage of firing first becomes much more important here.
We all watched the huge dense convoy into kyiv get bogged down and a single HIMARs launcher would have destroyed a huge fraction of that. The initial Russian attack was with concentration and did catch Ukraine mostly by surprise but could not achieve goals.
And of course, this is also about ww1 issue with mobile warfare. Even if you broke though the front, which has happened many times via concentration of artillery and high quality infantry, it is impossible to expend into exploitation because you need a rail-line to build up stockpiles of artillery shells and such to break the reserves lines behind that. Ww2 mobile warfare worked because an armored force can be resupplied by relatively small number of trucks and a narrow secured front with short prep is needed for the supply method to work.
Ukraine mobile warfare can hardly work when trucks can be interdicted at range by huge variety of effective means thus that even if you massed and gained a breakthrough, it is difficult to resupply another force in time to maintain momentum, and of course partly due to low efficiency of Russian artillery as to demand even more resupply when armor proved insufficient.
neither side has significant mobile reserves
It is more like both sides have all the (defensive) mobile reserves possible. In WW2 reserves were stuck on foot, and low performance towed guns have basically zero combat capability in a meeting engagement with armor, thus once breakthrough happens infantry reserves just gets out maneuvered and overrun left and right.
In this war both sides are nowhere near exhausting fast motorized vehicles and will probably run out of population long before that. Add infantry with effective ATGM or FPV and they can act as reserve to block any breakthrough. Armor on the other hand have evolved into heavy tanks with poor strategic mobility relative to the defenses.
www.latintimes.com
