Depend which countries you mean. There's a whole nexus of reasons why European nation spend less primarily because until the mid-2000s Russia did not seem a threat (even the US decided it needed to redefine NATO as a general alliance not an anti-Russian one) and European nations mostly do not have defence pacts with countries all over the world. The US has voluntarily promised to defend South Korea, Japan, all of South America, Australia, NATO and of course Israel. It patrols Taiwan to ward off China and maintains bases in basically every country. It has a sort of indirect strategic alliance with Iraq and Saudi for obvious reasons.
Anyway, this is all voluntary. It's not a bad thing and I'm sure Taiwanese people and Iraqis are happy for the support. It's questionable whether Australia and Europe need the support but it all comes with a heavy price. Australia doesn't want to guarantee Israel's safety - it has no interests there. France doesn't have an interest in Japan's safety so doesn't spend money getting involved.
My point is that it's silly to suggest other Western nations should match US spending because they haven't got the same obligations in East Asia and the Middle East as the US. They've intentionally stayed out of these alliances because maintaining them is expensive and has little benefit. In other words if the wants to continue to maintain interests all over the world and defend them too then defence costs will never go down regardless of what others spend.
Europe, to take an example closest to [my] home, faces serious threats from terrorism and Russia and that's about it. Taking Russia, it being the more serious and long term of the two, we can compare. Britain and France alone spend more than Russia on defence even today. Add Germany and Italy and they spend double what Russia spends. Add the rest of the EU and they spend 4 times what Russia spends. [Take spending over the last 20 or so years and the EU has spent over 6 times what Russia has in the same period.](i.imgur.com/hJzAOXd.png) France alone has spent more than Russia since 1992. We have yet to see the effect of the increases in defence spending in Germany, Poland and Eastern Europe after 2014 so the figures I've quoted me get even less favourable for Russia in the future.
Europe can defend itself but the US has interests in opposing Russia, in being friendly the Europe (often it's customers wrt military equipment) so it spends resources to defend Europe despite the fact that that its not necessary. All these countries have a mutual defence pact outside of NATO so a war with one EU nation is a war with all of them.
Again my point is that this is US policy not a necessity forced on the US unwillingly.. It can be so frustrating being from a European country when some (normally right wing) politician/lobbyist/'expert' makes some claim about how Europe is doomed to destruction because we don't match %GDP defence spending but it ignores the lack of commitments Europe has and the colossal number of commitments the US pays to maintain.
Europe can defend itself and its immediate interests without help and that's what matters to the people who live in Europe. It may not be the prime concern to defence strategist in Washington who think about US interests in militarily powerful allies in Europe but they don't, thank goodness, have a vote.
