Well, there are a number historical reasons for why Europe relied on the US for its defence and why the US was totally OK with that.
(1) Europe doesn't exist as a nation: it's like the US (states) without a federal government. That means it doesn't have an integrated defense industry or an army, which in turn means it doesn't have the economies of scale that enable the US to be a superpower. For a very long time this was considered to be in the US interest. US leadership was unquestioned, which had its use. Take e.g. the industrial side of the F16 and F35 fighter planes: those benefited (and continue to benefit) quite a lot from European clientele. The same holds for other types of weaponry. We're talking about 80 bln. in arms sales in 2023 and about 97 bln in 2024 to US allies (see e.g. here: https://ancillary-proxy.atarimworker.io?url=https%3A%2F%2Fwww.state.gov%2Ffiscal-y... and here https://ancillary-proxy.atarimworker.io?url=https%3A%2F%2Fwww.defense.gov%2FNews%2FN... ). Arms sales don't exist in a vacuum: it also represents a buy-in in the US as a partner.
(2) For the entire period from 1945 to the dissolution of the USSR in 1991, the US and Europe had this bargain: the US provides the military-industrial muscle and the nuclear umbrella, Europe provides the battlefield, about half the manpower, a robust first line of defense plus 100% of civillian casualties in anything but an intercontinental nuclear war. The US had a very clear interest in this tate of affairs, like e.g. keeping the USSR bottled up in a land-locked environment without practical (non-blockable) ice-free ports en a big counterweight to USSR expansion.
(3) For that reason the US had (for that period) been very much against Europe getting anything like a European army or an integrated arms industry. All that changed only when it realised that China had become an economic and military rival. With the Warsaw pact disbanded and Russia no longer a threat, US interests in Europe became more diluted and its commitments there more of a burden.
(4) Europe is indeed ramping up its spending, but there is of course a lead time (of 5-10 years or so) before it can get its 'own' military up to scratch. Withdrawing support before that time means risking Russian expansion once again. That could be an issue if Russia e.g. regains the industrial muscle and mineral resources of the Ukraina.
(5) Currently the situation is that the US has put Europe on notice to get a move on with financing its own defense needs and those of teh Ukraine. And this is having an effect already. Just for the record, US aid to the Ukraine is about 183 bln. by the end of 2024 ehile the EU has contributed slightly more than 50% of all military aid to the Ukraine to date (see e.g. here: https://ancillary-proxy.atarimworker.io?url=https%3A%2F%2Fwww.bbc.com%2Fnews%2Fartic... ). There is more to be done, but the figures mentioned by pres. Trump are outright misleading (as usual).
(6) It goes without saying that this development will (over the next 10 years or so) lessen US influence in that part of the world. With that withdrawal of influence there may also be a shrinkage of available military bases, and with it the ability to 'project power' abroad. That in its turn will further contribute to lessening US influence in the world. Whether or not this is desirable is another question, but I think it should be factored into any strategic decision makeing (which in my opionion currently is not the case).