You're kind of missing the point. I'll probably bungle this analogy, but why is it important to a furniture startup that a master carpenter can put together Ikea?
Because you’ll be a force multiplier making the IKEA builders have nicer materials and surface finishes, instead of making really nice furniture pieces that get thrown away on every redesign. Same reason to want a .NET backend dev that plays well with others, instead of a lisp dev with a perfect reimplementation of their own web framework, but going across stacks or teams requires onboarding on the tech instead of just the domain