I had this question why isn't Zoom giving the "Join from browser" option in the first go? Most of the time I would simply click the Launch button multiple times to get that "Join from browser" link. It's been on my mind forever.
Why Zoom doesn't give join from browser option by default?
Probably because they want you to use their desktop app.
Why's that? That needs some speculation, for which I'll step back a little.
The generalized question here is "Why do companies want me to use their dedicated (smartphone or desktop) app instead of just using a (mobile) web page?".
Broadly speaking, web apps are limited in various ways: Because they can't hook into the system directly most of the time (although things like WebUSB and WebGPU are being developed), developers of web apps need to hope that the browser's implementation of whatever it is they're doing is good enough. In addition, developers need to be cautious about extensions and addons potentially breaking their web app, and different browsers supporting different features.
Because of all these things, there just is a practical advantage for certain apps to use the desktop variant of an app, even if tools like electron make it easy to keep the basic functionality available on the web. And if the feature set of the web version is so limited that it makes your app look inferior to another app, there's a strong incentive to nudge the user to using the desktop app instead of the web app whenever possible to the product owners.
One feature that stands out as being available to desktop apps is the ability to make itself more seen: It can place itself on your desktop, in your start menu, onto your task bar and even into the autostart. Web apps have a much harder time doing that and if they fail to convince you to do it, they'll just end up deep in your bookmarks.
Zoom in particular isn't just an online meeting thing anymore, it is morphing to become an entire groupware with chat, email, calendar and other collaborative features. It likely wants to be a desktop app you have open all the time and have you organize your work life around it. And to achieve that it uses all help it can get, and there's just more of that available if you keep using their desktop app.
So, does this behavior strictly improve the user's experience? From a certain angle, you using Zoom for everything improves the value Zoom offers to you, so encouraging you to use their desktop app sort of improves the experience you get out of Zoom. From another angle, you might call this an annoying dark pattern, or a special form of marketing. But in any case it's very likely very good for Zoom to handle it like this.
Because the browser is limiting, they will have more control and features with their native offerings.
I have heard of "native-first" as well, a method of developing web apps where you try as much as possible to move users to your native offering first then have the web app as I backup. Reddit, I believe does this. As soon as you pop onto their site on mobile they offer their mobile app for downloading. But I couldn't find any articles using the phrase "native-first", so please take this last paragraph with the largest metaphorical grain of salt.