imo because addons shouldn't be a mandated "you MUST get this" thing, especially not useless stuff like instruments
add-ons like fillcan and new duplicator are one-step-below-mandatory QOL mods that everyone
should have. maybe not instruments, but he chose to include them because they're popular, fun and have a high demand from people who want to know how to install them.
this game is
made by add-ons. you'd be doing it a disservice to not include ones that make building and designing in this game far more user-friendly and fun.