Because you need 20x the JS to do the same thing and it’s still not hardware accelerated. These new CSS properties are well supported and will only get better.
<span onclick="
banana.
classlist.
toggle('hidden')
">click</span>
<div id="banana" class="hidden">loris</div>This is the HTML version, it's not susceptible to halted execution, and it is accessible.
<details>
<summary>Click</summary>
loris