"Template" might be a misleading term. Reusable blocks are more text snippets, more like reusable content with fixed layout.

If you reuse a block, like a call-to-action button 30 times around your site and for 31st time you change the URL - the change will propagate over all previous instances of your block.

There is a step in between to prevent this: Convert the reusable block to stand-alone-block first, before making an unwanted change.

Block Templates with placeholder content are still reserved for Custom Blocks