some reasons that I can think of:

• performance improvements (e.g. JIT)
• new standards (e.g. WASM)
• vulnerabities
• new features (e.g. web engines weren't always sandboxed)

Some of the new features most people aren't aware of us that I used recently :

• WebXR, make a Web page immersive and work in the browser of VR/AR headsets, e.g Meta Quest, Lynx XR1, Apple Vision Pro, etc
• WebBlueTooth, connect to a BT device, e.g a Lego controller in order to move actuator, data from sensors, etc
• WebUSB, connect a device and update its firmware, e.g SmartWatch, mechanical keyboard, etc
• GamePad API, use a gamepad or joystick to play from a browser window
• Realms in JavaScript for "better" sandboxing, it's a relatively new feature of the language so the engine must be updated

So... sure none of that really helps to read a 2D Web page (like this one on Lemmy) but they pretty much all help to achieve better cross-platform support. By using the Web rather than native to connect to hardware then it is instantly delivered without having any OS specific driver to build and install. Practically speaking it does make the browser increasingly complex but IMHO it is worth it.

PS: I probably also used some modern CSS so there also the engine (which is ridiculously complex by the way) has to be updated too.

HTML used to be a pretty set standard, maintained by the W3C. HTML5 was retired in 2018 (5.2 in 2021). Now it is a Living Standard that changes often and is maintained by a consortium of browser vendors.

It is also not the only technology being changed.

There are features that constantly get added. It's not only HTML (maybe the html part is stable, I don't know), but there's CSS and most importantly JavaScript.

Also, browsers don't always follow the standard exactly. Some features get added that aren't in the standard.