Seems like a mix of things:
• The culture of incentivising employees with equity stakes is mostly a US thing. It's not widely practiced outside the USA. ARM is a rare example of a British tech firm that gave out equity to early employees, and not coincidentally, ARM was (up until it got bought by the Japanese) one of Europe's tech success stories.
• Lack of a comparable venture capital ecosystem of the type that came out of the success of Silicon Valley.
• The EU single market is (a) relatively new and (b) overhyped. It's still much easier to sell across state lines in the USA than it is to sell to the EU market, for all kinds of reasons. Of course the single market made it a lot easier but still not as easy as when there's a single language, a well established federal-level trading code that's been around hundreds of years etc.
• During the time the big US tech firms were establishing themselves and taking off (the 1980s), Europe was riven with hard socialism and even dictatorships. Spain was a dictatorship up until 1982, and they then elected socialists. France has famously strict labor laws that make it extremely risky and hard to try and grow a startup (which is already risky and hard). Italy was bogged down from the 70s-90s and arguably still today by heavily broken interventionist labour market, and terrorism/political instability. Germany didn't liberalise its labour markets to something approximating US standards until the Hartz IV reforms in the early 2000s and of course was split in two with one half being another dictatorship until 1991. During the 1980s the UK was busy recovering from a near-fatal economic meltdown caused by confrontations with hard left unions, that had rotating power cuts, three day weeks, garbage piling up on streets and an IMF bailout. Thatcher spent the whole of the 80s trying to bring the UK back to western liberal standards of growth and prosperity, so it wasn't an environment conducive to making a Microsoft or an Apple.
So it's a combination of US style VC fuelled tech ecosystems taking decades to grow, lack of equity-based incentives, most of Europe not having had sufficient political stability during the time companies like Apple, Microsoft, Intel etc were becoming established, and large parts of Europe even today still having radically more socialist governments and economies than the USA with the concomitant difficulty of growing any kind of startup at all, let alone high risk/high reward tech startups.