It's because proof-of-stake is fundamentally different from how proof-of-work operates.

The fundamental problem that all blockchains need to solve is something called the Byzantine Generals Problem. A blockchain needs to consist of a list of transactions that everyone agrees on - everyone needs to be able to know which transactions are part of the list, and what order they appear on that list. But there can't be any central "authority" making that decision, it has to be done in a completely decentralized way.

The way proof of work does it is that it requires people adding transactions to the list to do some extremely expensive calculations and attach the results of those calculations to the transactions that they're adding. Anyone can do those calculations so there's no central authority, but the costliness of the calculations means that once the transactions are added it becomes just as expensive to create a substitute set of transactions. So everyone ends up agreeing on what transactions were added because it would be unfeasably costly to "fake" an alternative history to the blockchain. This means it's impossible to make a proof-of-work chain that isn't hugely "wasteful", because the waste is the point of it. It has to be costly for it to work.

Proof-of-stake takes a very different approach. It solves the same basic problem - determining which transactions are part of the chain in a decentralized manner - using some very fancy cryptography that I have to admit that I don't fully understand. But instead of proving that the transactions you're adding are "trustworthy" due to proving you've wasted a whole lot of resources adding them, you do it by putting up a "stake." You lock a big sum of money in your cryptocurrency staking account and essentially make it a hostage to your good behaviour. If you put up a bad transaction you can lose your stake. So under proof-of-stake there's simply no need to burn huge amounts of electricity.

Monero uses a proof-of-work algorithm like Bitcoin. The reason Monero doesn't use anywhere near as much energy as Bitcoin is simply because it isn't worth as much and so not as many people are mining it. If Monero was worth as much as Bitcoin the energy usage would rise to become comparable.