“That’s why this is a WWI style trench warfare right now.”
One reason.
A bigger one probably is that neither side can organise a multi echelon attack, to break through and exploit.
The Ukrainians lack materiel, artillery and armor, They managed such “maneuver warfare” type attacks last year, against depleted and scattered Russians without a continuous front. But something more is required now that there is a continuous front.
The Russians failed last year when they had the means but not the skill or preparation. And they failed again at Izyum. And now because of losses of equipment and cadre, they seem even less capable. To add, the Ukrainians have means of long range interdiction (HIMARS) that keeps them from concentrating for a multi echelon attack.
Their best chance was on day one when they went for Kyiv. Even if they were able to take Kyiv, they didn't have enough troops to occupy the city.