As the NVA/VC knew, especially with the US withdrawing troops, they still had to continue to attrite the US/SEATO. To help insure the US will not think about staying any longer, etc. They saw the effect of the war was having back in the US. So killing off as many US/SEATO troops was certainly to their advantage.
And by that time the US had unit made up of many draftees. Many of the "professionals" had served more than one tour already. Plus the US had about 8 Divs + worth of troops in Europe, 1 Div in the ROK, etc. Those all have to be kept up to strength as well. Many draftees did not ever get to Vietnam.
Many of the US troops in Vietnam served and fought well in '71-'72. However, the quality of these troops were in some cases someone the US would not have drafted if they didn't need the numbers. The standards were lowered, per se. But many fought well and died regardless.
Even Giap thought the '72 "Year of the Rat" Offensive was not really needed at that time if at all. But he was overruled by others in the North's leadership hierarchy. The NLF/VC had been significantly attrited when push forward during Tet '68. By '72 they could only really deploy about 3 Rgts. And were fleshed out by NVA in some cases.
But bottom line the North's leadership wanted the US/SEATO out and the South Vietnamese military destroyed. So the pushed their forces forward regardless of the losses they would incur. And just like during Tet the NVA/VC took heavy losses from the US firepower, etc. But they want the Vietnams united under their North's control.
Regardless what it would cost them in men, equipment, etc. And in many cases the NVA were taking on ARVN units without US support. And history shows in many cases the ARVN were not up to the task of saving their county from the North's Communist domination.