Because, like all Electron-based apps, it's really just a Chrome instance in a Slack wrapper. Likewise, Slack is currently a big drain on batteries. Even consider running Safari for normal browsing and Chrome only when you need it for heavy Google work - that's what I do. So, if you don't need something that's Chrome-only, consider running Safari. Some tests have shown turning off Chrome can save you up to an hour in battery life, which is quite significant. Chrome is cross-platform and has become almost Internet Explorer-like in its desire to make the browser a platform. This is especially true for M1 Macs, which have four out of eight cores dedicated to power efficiency (the other four are for performance).
MACBOOK PRO BATTERY X 2017 CODE
Safari is better on battery life than Chrome because Apple only makes Safari for macOS and can code it to be as efficient as possible.