This is true. However, an attacking force(the enemy) does not usually spend hours preparing defensive positions.
Modern war is a war of mobility. Especially on offensive. You move and hide to avoid artillery (unless you are speaking about that weird Ukrainian-Russian war that looks like unholy mix of WW1,WW2,cold war era conflicts and spec ops playing counter-strike in the real life)
That movement also stops when it meets defenders or another obstacle.
No it doesn't. You don't just stop maneuvering when you locate the enemy. You try to outmaneuver him. Flank him or surround him or cut him off.
If it only worked all the time. Also, cutting off a unit may not have immediate effect on its abilities.
A possible scenario: a mechanized platoon advances on road and suddenly meets defenders. Terrain: woodland, average visibility 200 meters, more near the road. There arent many defenders, its just a delay action, but they have mines, mortars and AT weapons. The lead recon squad of 2 BTRs is let through before its destroyed and the first elements of the main body come under fire. The width of the attacking formation in a space limited by water and difficult terrain is 900 meters. The formation stops, engages, and notices it cant just push through easily. Another vehicle is lost. This has taken 3 minutes. It takes a minute to pass the information up the command chain. Another minute or two for the HQ to orient and make a decision, and another minute for the order to disengage, regroup and flank using the difficult terrain together with a formation behind, while artillery and/or air units are called to fix the enemy, to be received. Just disengaging may take more than a minute.
From somewhere around the 2 min mark they can expect an artillery barrage on their heads from a unit that was already prepared for fire missions to this location, 2 to 3 minutes longer if the artillery needs to move from cover to firing stations first. Likewise, this attacking unit may have its own support in similar readiness, but likely further away. 3 times 18 guns firing twice is brutal at the receiving end even if the hit area covers the targeted enemy unit only partially. 155 mm DM662 airburst shell has 49 submunitions, each with lethal anti-personnel shrapnel range of 10 meters and over 100 mm of steel penetration(on direct hit). HE shells are as a rule rather than exception timed to detonate in the air as well, because the best cones for shrapnel spread are to the sides and below, slightly towards the direction the shell came from.
There are plenty of studies on available on artillery. Using the formula available
here, page 9, for semi-hard targets, just the first shots in the situation described above, target area being 900 x 900 meters, would result in statistical probability of each man and armored vehicle(but not MBTs) in the target area to die or become incapacitated at 36% chance. Airburst rocks. P = 1 - e^(-(314*49*18*3)/(900x900))
Modern warfare is indeed about mobility, but also about firepower, and artillery is still very good at delivering the latter. A "defender" is often also able to attack(in qualitatively symmetric warfare), and units no longer form a front but rather mobile fighting "pockets".
EDIT: My math sucks. Its 64%. Assuming just half the shots hit the area, lethality is still 40%. Swapping that airburst dual purpose cluster to HE(wounding radius 25 m) and its 12%, and more in wooded terrain as shells splashing against trees actually increases their effect. Let the HE shells explode in ground contact only and it drops to mere 2% or even less if the fuse has a delay.