> but I wonder why there’s no library that uses both

Sciter (https://sciter.com) uses both, as internally as on user's level.

For example you may want to define this:

   var bodyElement = document.body;

   bodyElement.paintForeground = function(graphics) {
     ... draw something 
     ... on top of <body> content 
     ... using the graphics 
   }

This allows to benefit from both: retained mode (HTML DOM, layout cached) and immediate mode rendering - paint handlers on DOM elements.