Ukraine has one of the strongest militaries in Europe. This whole "they couldn't even beat puny Ukraine" line I keep seeing is entirely too haughty for my liking. Their gear is less state-of-the-art, sure, but many European countries lack vital components of a functional military altogether. Including logistics and coordination of joint efforts which the Americans have until recently been doing.
Sure, no need to panic yet, but certainly a need to get a move on and actually respond proactively to make up for gaps, and respond jointly, to ensure that it's not going to be a matter of small countries getting steamrolled one by one.
Probably many greedy reasons, but my personal favourite speculation: annexing Greenland surrounds Canada and stops any potential aid by its NATO allies in case of an invasion, since annexing Canada is one of the stated objectives of the US now.
In terms of strategy for actual national security, they already got all the access they wanted, if they wanted more all they had to do was ask. If they're the ones doing the attacking of a common ally, though, they wouldn't get that access. So it's only of added strategic value to annex instead of maintaining the alliance if the goal is to attack members of the alliance.