Most software runs locally. "What you say, jerf? It's 2022, man, keep up." By that I mean, in terms of resource consumption, most software uses mostly local resources. Your Slack server footprint is tiny compared to the resources eaten by your slack client. Websites explode in RAM and CPU compared to what it took to serve the resources to your browser. Streamed video is expensive to encode once, but will be decoded somewhere locally millions of times. Especially for systems large enough to be worth optimizing and where they're not some guy's first Ruby on Rails project (built before he really got how not to make 15,000 queries per web page) or something. Servers for Slack or Office 365 or a streaming video service are certainly Big Iron and expensive, but compared to the sum total of resources sitting on the client side they're tiny.
Charging by the second for Office 365 wouldn't be smart, because their marginal costs aren't all that related to how many seconds of use there is. (They're certainly correlated, obviously, but I can find you signals that are much more strongly correlated than simply seconds of use, like document size and how many times they reload the page.) So you get all the disadvantages of your user sitting there thinking "How fast can I get this done? Every second I delay I'm paying more!", but you're not saving the resources or anything.
Pure time-based costs make sense if you're selling truly time-based resources, but are questionable in other places. For a lot of services, if not the majority, the true nature of the marginal costs are really hard to divine, and the users don't much care anyhow, and the marginal costs are much smaller than you want to charge anyhow, so it's much easier to just charge them $10/month and get on with it, rather than do all the complicated math to discover that this user's marginal cost was $0.33 this month.
(If my choice of example numbers confuses you, be sure you know what a marginal cost is: https://www.shopify.com/encyclopedia/marginal-cost . There's a lot more to pricing a cloud service than marginal costs. With modern hardware, networking, and even a cursory stab at some optimization by the developers, the marginal cost of a lot of useful cloud services can be tiny. There's useful cloud services out there where the marginal cost of a user is probably less than a penny per month. All the other costs can be quite non-trivial, though!)