Why don't we have a Mishkan today?

Before Jews had access to the Temple Mount they used a temporary structure in Shilo based on an earlier temporary structure constructed in the desert. 

Why aren't Jews interested in temporary structures anymore?

 We don't have access to the Temple Mount today. A rebuilt Temple is, for the moment, impossible. So why don't we set up a temporary Mishkan like structure to temporarily hold us over? 

The answer, obviously, is that animal sacrifice is ridiculous and no one really wants to see it return.

Also, Judaism today is controlled by Rabbis. If we built a new Shilo some control would have to be shared with Kohanim. And no one likes to share control.

Also, the people profiting from major Jewish pilgrimage sites (which operate like a temporary mishkan, with financial sacrifices rather than animal sacrifices required) have zero interest in sharing revenue with a new Shilo.

(The actual Messiah will run into the same problems when he tries to replace Uman with Moriah and installs a young, fit, Kohen as spiritual leader in place of our doddering nonagenarians)

To all those saying there are rules in Nach against bamot I say:

I refuse to believe our religious leaders couldnt find a work around if motivated 

Yochanan ben Zackai certainly managed to work around several rules.

The very fact that people are shrugging their shoulders shows me there is no appetite for animal sacrifice. 

People don't shrug their shoulders and give up immediately when it's something they really want. 

When it's something people really want (like Covid Minyanim and Weddings or sneaking into Uman for Covid Rosh Hashana) there are always people who can make it happen.