I don't know if calling it "much easier" is really fair. We see how easily humans break (and can be broken) and become evil all the time. Corruption is one of the main themes in the game. But, maybe because we are human and because there are so many human characters that it's easy to say "not all humans", we're more willing to not fret about that all the time. We've also seen quite a few demons, the nasty and dangerous side of the Fade. Meanwhile, we've met less than a handful of spirit characters, and are only exploring a positive and somewhat more in-depth look at them for the very first time. So it's easier to be wary of them and the risks attached to them -- their alien nature and little in the way of positive interactions with them don't help the anti-spirit bias, especially when humanity (which many are inclined to view as superior anyway because we are human) is the other option.
Solas explains to Inquisitor that spirits able to reflect reality (namely those with developed personality) are fairly rare, so I don't think we'd be seeing many of them. But those we know... I mean, just look what happens to both Cole in Calling and Wisdom in Inquisition. They got fairly easily twisted or at least made severely confused about their identity.
I think the game implies pretty heavily that spirits are more fragile and susceptible to corrupting (or "corrupting") influences than humans - they're creatures from realm shaped by will and emotions and so they can be very easily twisted or adapt to human expectation (as explained, again, by Solas). They are also pretty simple/naive/childlike creatures who probably just don't have defense mechanisms against such influence.
Spirit and demon motivations are also fairly simple and so are their lives, if they're left alone. They're not really interested in much aside from what they stick to - and it's Cole who said that. Even him, a more complex spirit, just dwelled in the Fade and did his thing until Cole's hurt brought him to Thedas - according to banter, he wasn't even aware of existence of Wisdom spirits, or areas of the Fade he didn't originate from (the domain of Nightmare freaked him out, seemingly not just because he entered it as... uh... semi-human).
It's different for humans and other races in Thedas - they live in a world were such simple existence is pretty much impossible. Hence they're beings who are more complex. That makes them less pure than spirits, but also less prone to total corruption or sudden shifts in one's personality. Oh sure, they're also more prone to evil or gray-area behavior, but at their core they're actually more resilient to external world as well as will of others. I also wouldn't call what we see in games as humans being "easily broken and made evil all the time". Most of them are either small-minded, desperate or do things they think are justified - while most demons (aside from those most powerful ones) just want everything destroyed or twisted around a negative thing they reflect, with little to no regard to anything else.