(Disclaimer: the following is just a personal thought, and isn't intented to display a defense for either side.)
The question you all should ask is the famous "cui bono?": who profits?
Right now, Zelensky has to deal with Trump... who is known to be very receptive to toadying as well as vindictive towards those he thinks are his "enemies". Zelensky has nothing to gain by going against Trump right now.
There's also not much danger for Zelensky in going against Biden right now. Any "investigation" will be as deep or shallow as Zelensky wants... and if Biden wins the Presidential election, Zelensky can still drop any investigation, or declare it came out in Biden's favour.
On the US side... what do you think would happen to Biden if the Ukraine decided there had been any kind of "corruption"? Do you think the USA would extradite one of their citizen to a country like Ukraine to stand in court there? When it is heavily opposed in doing such even in cases where there are clear and evident law violations?
Let's say the USA would do so. What do you think would happen if it became practice to hand over your political opponents to foreign nations for criminal punishment... foreign nations that the current government of the USA has some form of leverage over?
Do you think Republicans will hold the reigns forever? If they hand such a weapon to the Democrats, and continue to make enemies of the Democrats in the way they do now... do they really believe no future Democrat administration will do that in turn to them?
And I don't believe Trump is even interested in having Biden convicted in any form. Biden under scrutiny, Biden under investigation, Biden in any form connected to something uncouth is much better for Trump than a Biden investigated... and potentially cleared.
Look at Trump's approach to Hillary Clinton. She is "crooked". She is "criminal". "Lock her up!" was the most popular chant in his election campaign. He declared that if he won, Hillary would go to prison.
And after his victory? Not a peep. No investigation. No court case. No prison time for Hillary. I couldn't find the quote... but he deliberately said that he wouldn't bother with such things any more.
Why? Because a Hillary that he can call out for being "crooked" and "criminal" every time he wants to is much more useful to him than a Hillary being cleared in court. Or even a Hillary convicted in court... because he knows he could face the same fate.
It's the same with Biden right now. A Biden that he can call "corrupt" is useful for Trump. If Biden was to drop out of the race today, you wouldn't hear anything about it anymore.
If said it before and I say it again: I don't care about Biden, at all. I don't have any stakes in the US elections.
I am just amused (and despaired) by those who try to make Trump - of all people - into some kind of hero.