Because the debt is not sustainable. It will leave the Greek economy with huge interest payments that will cripple its economy for decades to come.
It is in Greece's interests to at least significantly restructure the repayments (much longer terms, with 0 interest on much of it for significant periods). Ultimately, they can do this unilaterally - "default" - in which case they might as well give themselves the best possible terms.
The vast majority of debt is owed to EU member states and institutions, and France and Germany in particular. It suited France and Germany for Greece to take on massive public debt in a bailout in 2010, because a good chunk of that money went to repaying *French* and *German* banks which risked becoming insolvent and collapsing otherwise - as Greece didn't have the money. The 2010 bail-out was as much, if not more, about saving *French and German* banks as about bailing out Greece. Indeed that bail-out broke IMF rules, but the IMF made an exception because of the exceptional systemic risk to global finance and the fears of another Lehman Brothers like collapse.
The sane solution for Greece in 2010 would have been to privately negotiate debt restructuring with its creditors, and unilaterally restructure payments where ever agreement couldn't have been reached. However, Greece was instead heavily pressurised by the rest of the EU to *not* do the sane thing, but instead take on *more* debt, because some of its creditors might have gone bust had Greece done the sane thing. Greece did this under pressure, and took this on to *save European banks*. Its own banking system got about €48 bn, to recapitalise, and its government got perhaps €20 bn to €30bn for operational needs (though even some amount of that went on financing costs).
Greece has been made to pay, very heavily, for the mistakes of the French and Northern European banking sector. Further, those who were *least* responsible for this crisis in Greece have been made to suffer the most - the poor, the young. This is fundamentally unfair.
Greece acted in solidarity with EU partners in taking on this bailout of, largely, *OTHER* European banks. The bailout was simply too big to be sustainable for the Greek economy, which has collapsed amid one of the largest recessions ever seen in the modern world. Instead of acting reasonably and recognising the solidarity the Greeks showed in 2010, several EU nations have - cowardly - instead tried to paint this as all the fault of lazy Greeks, which is an objective falsehood.
The EU *needs* a deal with Greece, because, as with all the previous deals, they are protecting *their own* banks, above all else. The Greeks now are saying any such deal also has to stop punishing them.