I don't know if there's a reason why this wasn't added before, but I agree with the reasons over at #36234.

There was a desire to delay surfacing this block support until we have a better way to manage custom fonts. A summary from a chat with @mtias can be found in #34064 (comment)

My understanding was we'd only keep font-family as an option for the blocks that already had opted into that support and ensure it wasn't displayed as a default control within the typography panel.

There was a desire to delay surfacing this block support until we have a better way to manage custom fonts. A summary from a chat with @mtias can be found in #34064 (comment)

My understanding was we'd only keep font-family as an option for the blocks that already had opted into that support and ensure it wasn't displayed as a default control within the typography panel.

Should we revert this then? I wish we could have this in the block markup even if the UI doesn't show it to the user

I think we should revert this based on previous conversations linked. Let's please get more reviews and ping folks working in this area before merging, we are happy to help.

Based on the committed file, a temporarily hack:

add_filter('register_block_type_args', function($args, $name) {
    if ($name == 'core/paragraph') {
        $args['supports']['typography']['__experimentalFontFamily'] = true;
    }

    return $args;
}, 10, 2);

or, to show up as default controls:

add_filter('register_block_type_args', function($args, $name) {
    if ($name == 'core/heading') {
        $settings['supports']['typography']['__experimentalFontFamily'] = true;
        $settings['supports']['typography']['__experimentalDefaultControls']['fontFamily'] = true;
    }

    return $args;
}, 10, 2);

will do the trick for now until official solution.

@alan-la-chen-478 Thank
I tested your small hack but I have not core blocks in the filter. I have WooCommerce blocks and Gravity block but I don't see core/heading and core/paragraph. Do you know the reason ?

Edit:

I found a solution. I did it in JavaScript and it works

const { addFilter } = wp.hooks
import { set as _set } from 'lodash'

function addFontFamillySupport(settings) {
  const names = ['core/heading', 'core/paragraph']

  if(names.includes(settings.name))
    settings = _set(settings, 'supports.typography.__experimentalFontFamily', true)
  
  return settings
}

addFilter('blocks.registerBlockType', 'koa/font-familly-supports', addFontFamillySupport)