1) SMBs don't necessarily hire that many software engineers. Hell, even tons of large companies barely do. I work at a multinational (100 countries) which mostly buys off the shelf software, and uses basic tools like Excel to run a ridiculous amount of internal business processes.
2) Non-software companies tend to hire software companies, not software engineers. Due to the economies of scale that comes with companies producing and selling (standardised enterprise) software, salaries of software devs aren't that big of a problem necessarily. A company like Trello for example built a collaboration tool with 10s of engineers that has something like 100 million users. The numbers aren't exact, but the order of magnitude is.
Beyond that, you'd be surprised how large the nominal income differences are for US/EU companies, individuals, households etc. The extra money doesn't translate into much better quality of life in the US (due to the crappier working hours, the no legal holiday rights, crappy parental leave, no cheap universal healthcare or tertiary education, poor public transport, no walkable cities, inequality, more energy-intensive economy blablabla), but purely on a nominal dollar basis the US really outearns Europe by quite a large margin.