By default, add-on toolbar icons are stored in the Extensions button (looks like a puzzle piece) on the toolbar. This is to stop add-ons from cluttering up your browser.

You can pin some extensions to the toolbar by pressing the Gear icon next to the add-on and selecting "Pin to Toolbar". However, not every add-on can be pinned to the toolbar. It depends on what functionality the add-on developer has built into the add-on.

For more information, please refer to the Manage your extensions using the extensions button in the toolbar article.

Hope this helps.

Because those extensions probably don't need shortcuts, they just run in the background or add buttons to the toolbar instead.