The simplest explanation is that it is stupidity. They ignored the lesson that Ukraine learnt about the difficulty of mobile, integrated attacks and went all in on the expectation that they could storm weak Ukrainian positions at Adviivka easily.
The reality is otherwise but they can't admit to their stupidity. To do so would indicate weakness.
The propaganda channels on YouTube were saying Russia was going all in with this attack and would crush Ukraine.
The situation for Ukraine there is precarious. The distance to cover isolate the Ukrainians is not far. There is only one road in for supplies once the weather worsens.
An alternative view is that Russia is continuing to attack in Avdiivka because they are achieving their objectives albeit much more slowly and at a much, much higher cost in terms of men and materiel than any Western army would ever countenance.
As high as the cost it for Russia, it's a cost that they're apparently willing to bear - presumably because they have plenty of men and equipment. Those men may not be considered adequately trained by Western armies and the equipment is vastly substandard but it's sufficient for the "meat wave" tactics that they're adopting.
So what if they lose 100,000 men taking Avdiivka ? If Russia can withstand those kinds of losses then they may consider it worthwhile - especially if they're just waiting for a Trump victory and all that means in terms of (withdrawal of) US support.