Capturing Ukraine doesn't prop up the Russian economy either. Trading with Ukraine would. Trading, to the extent possible with poor countries, helps the economy. Blowing **** up doesn't help anyone's economy.
And yet, Putin decided to blow **** up.
Conquerors don't care about the economy.
He didn't expect there to be any real resistance, that's why. He went in expecting to roll over the Ukrainian military in less than a week and install a puppet government.
Unfortunately that hasn't happened. Russia is making really slow progress in the places it's making any at all, and it's becoming increasingly obvious that the Russian military cannot take and hold Ukraine.
Normally this would be the point at which someone who isn't in Putin's spot would think "Oh ****, I need to stop this now so I can trade again". However Putin is not in the same position that, for example, Joe Biden or even Trump would be. Putin's entire schtick is being a Strong Man
TM. A Strong Man
TM that uses military might to get what he wants and bullies The West into letting him do what he wants. He can't back out at all because if he does he loses his entire image in one go and looks weak. It's bad enough when a leader in a Western Democracy looks weak, but for a Strong Man
TM dictator? It's suicide, possibly literally.
So now Putin can't back down or he will look weak to his people, both the average Josef on the street and the people who actually matter, the Oligarchs and the military leaders. So he keeps pushing forwards instead, and as a consequence begins to look weaker and weaker and weaker...
Putin can't go forwards or he'll end up humiliating the Russian military, but he absolutely CANNOT go backwards because he will look weak.
He's basically damned if he does and damned if he doesn't, only if he doesn't (keep going) he's even more damned.