There has been some good answers, but I'm not entirely satisfied with the details, so I will add my own response.

Culturally Russia sees itself as outside the rest of the world. At the very minimum, an equal to historical empires of Europe or Asia, but part of neither. It sees the USA as an ethnic mongrel with no culture or history, and hates the US power it projects globally.

Russia sees the former Soviet Union countries as property of the Rus people, and NATO involvement as outsider influence in affairs that do not concern them.

Globally, the world values stability more than they value justice or peace. When Russia invaded Ukraine in 2014, it came after several other invasions of other former Soviet countries. There was little global response on any occasion.

Putin did expect the invasion to be fast and achieve their goals quickly. It was a mistake on his behalf.

This invasion was taken differently than any previous invasion because it upset global stability. Gas, oil and grain were traded openly with Russia and Ukraine and a war upset the market right when the world was trying to stabilise markets rocked by inflation, pandemic recovery and suppy chain problems.

The result was many countries around the world pledging military support. This was always older generation materiel which essentially costs those countries to maintain. It was the global equivalent of giving a homeless man the doggy bag you didn't want anyway.

Why did they do this? They wanted Russia to pull back, return to its 2014 lines and go back to stability so that global markets could resume. So they gave Ukraine just enough to defend itself, but not enough to win.

Why did they do this? Because the world wants stability more than peace. Of the pledges of materiel, almost none has actually come to fruition. About 1/4 of the armor promised has arrived that was promised. Ukraine continues to beg for alms (or in this case arms), and they do amazing things with the little they are given.

Western powers could arm Ukraine and it would win. They have had no problem spending trillions of dollars over decades to protect their influence. It does not in this case as the World is only just coming to terms that Russia will not stop just for stability.

Putin will cease to be leader if he pulls back. The Russian leader would be seen as weak, and the Russian culture loves a Tsar. Putin believes in luck and will continue the sunk cost in the hope that some outside factor or random event will go in his favor.

The West is already getting bored and tired of a war they aren't even fighting. There is a possibility that pro-Russian Republicans could regain office or power in the US. All Putin has to do is hold and eventually the West will even start telling Ukraine to capitulate to them.

Putin does not care how many troops he loses. Russia doesn't really care how many people it loses unless those people are from the cities. Russian culture dehumanises the poor and mixed ethnicities.

This current grinding stalemate is a direct result of world policy. The world supplies Ukraine with just enough so they don't lose, but not enough that they can win. In the meantime, the arms dealers are circling like sharks. India and China are cashing in on filling global supply gaps and taking advantage of Russias need for materiel frozen by sanctions. The hope would be that world leaders realise before it's too late that the only way Ukraine can win, is that if Russia loses.