As a former hiring manager at both Google and Amazon, I can respond at least about those two companies.
Firstly, getting in to Amazon is not easier than getting into Google. The hiring bar is not lower. It's different.
If Google had to boil their interviews down to one question, it would be “how smart are you?” If Amazon did the same, it would be “how productive are you?” Both value many other things as well, but that's the core of each.
From my experience, Amazon's interview process is more thorough and disciplined than Google's (and I trust it more) but neither is at all easy.
Secondly, Amazon is not universally seen as equally attractive as Google.
For those who value learning, being challenged, and getting satisfaction from getting things done, Amazon is a more natural fit, and is likely to be more attractive. Google is a bit “soft” in these regards, though markets itself very well.
For those who value lifestyle, perks, making their friends jealous, and being (and appearing) smart and being around similarly minded people, Google is more attractive. Google is all about being a great place to work (especially for software engineers), while at Amazon, employees are a means to an end. Employee happiness and work/life balance is conspicuously absent from Amazon's leadership principles, and that is evident in their reputation as a tough employer (see the infamous NYT article, which was widely discredited as anecdotal, yet contains elements most Amazonians will find somewhat familiar - particularly “Amazon is where overachievers go to feel bad about themselves”).
Most people fall into the second category, which makes recruiting at Amazon seem a bit tougher than at Google. Amazon hiring managers have fewer levers to use to attract people, but then, the levers they have work well on the people they want to attract. Google attracts more people, and hence rejects more people, leading perhaps to a reputation of being very hard to get into.