Steam IS DRM, yet people flock to it.
Ease of use.
Everything is one one spot.
Ability to just download again without looking for a disc.
Connecting with friends both with chat and gaming together.
As long as the value added when bound to DRM is greater than than a pirate's cheapness, they will deem it worth buying.
Also, the fact that Valve isn't a dick company like EA, Ubisoft or Apple tends to help matters.
Generally, when companies impose DRM - the DRM hampers the operation of the system. Like when Sony DRM'ed DVDs and invented Root Kits in the process. Any DRM that Steam uses does not detract from the actual gaming, it's a small thing that doesn't affect operation or performance. But in most cases, DRM schemes are all about affecting operation and performance - like DRM'ed music that won't work on whatever device you want to use it on, or DRM schemes that lead to bloat and massive download times, or DRM schemes that over-compress the music or video into some low quality glop that is worse that the garbage that passes for entertainment on YouTube.
DRM just ends up creating an industry for DRM removal tools or "piracy". And I can see that, because if say, I buy a CD that I want and it has DRM - I will certainly resort to whatever means it takes to remove it, so that I can listen to the CD on whatever device I want, with the highest possible quality, and without having to waste storage on my device. And I might, even if I do purchase, choose to "pirate" simply so I don't have to remove DRM myself, or to obtain whatever in a format I want to use.
This is an age old game - nothing new here. Every method that manufacturers have ever developed to prevent "piracy" has simply lead to more piracy - even if it is a hardware dongle hanging off a port - someone, somewhere will engineer a device to do the same.
Instead, maybe the makers should just put out something good, or a reasonable price - so that people will just find it easy and convenient to buy. It's like music - it is easy to go to a store and browse and buy, because if I have to download it, I might find it cheaper and easier to download from TPB than to download it from some bloated, overpriced online store where I am stuck with their crummy, low quality, DRM riddled format that only works on their crummy, proprietary devices.