How much would you guess your 4G plan would cost if it worked the same (i.e. no roaming, no data restrictions) from up where you are in Finland, down to Greece, and from Iceland over to Georgia? That's a rough approximation of the geographic area that the big four networks in the US cover, in terms of infrastructure and service.
I honestly don't think Finnish plans would be nearly as cheap (or as fast) if they covered the same amount of land as AT&T, Verizon, T-Mobile, etc. I don't think the carriers would be able to spend a disproportionate amount of their CapEx (infrastructure) budget on ridiculously fast speeds in the metro areas at the same time they need to pay to put cell phone towers up in rural fields to bring 4G LTE to non-metro areas across all of Europe (which is something AT&T, Verizon, T-Mobile, etc do in the US).
So when you say "one could expect better service", one thing to take into account is that US plans do include 4G LTE speeds in virtually every metro area in the US. That's like 29 Finlands worth of coverage.