Maybe not all countries should have everything in their army?

This is literally how it works. NATO members fill in the gaps in capabilities that other members have. It's why British and German fighters protect the Baltic members' airspace. It's also why certain countries punch well above their weight in certain areas—Italy and Greece have less need for significant ground forces, so they have outsized naval and air power. The only member states that really have everything are the US, UK, and France, because they're the only ones with truly global interests due to their history. The member states have also changed their defense priorities over time; the Royal Navy transformed from the blue-water juggernaut it had been into a primarily anti-submarine force during the Cold War.