Because every unit of electricity causes climate change and burns resources (even renewable sources of electricity - they just burn them slower). From a societal point of view we are dumping huge amounts of electricity and resources into a hole to accomplish nothing that couldn’t be accomplished with a database and a trusted third party at a billionth of the cost (or less).

The vast majority of transactions are speculation on what other people might pay for a bitcoin (i.e., a line on a spreadsheet). And even then, that speculation and trading often occurs on secondary markets which rely on trusted third parties - thus rendering the entire ordeal even more pointless.