I understand your point because I have a 5D and I, too, was disappointed. However, there are a few things to think about and realize.
First, you're shooting upward at F8. Without knowing exactly which lens you used, where the AF point(s) hit or whether the camera was on a tripod, I'm thinking DOF is the main culprit here because the AF point(s) appear to be up higher. One way to fix that is to manually set your AF point and shoot at F16 or beyond. If I want absolutely everything in focus, I usually max out the F-stop for whatever lens I'm using.
Second, your screenshot shows a preview of a JPG. To acheive the best results, shoot RAW and edit RAW. You can then edit with the Canon software (which is free and really not a bad in a pinch) or Adobe Photoshop or, preferably, Adobe Lightroom. You also need to understand that sharpening a JPG is all about approximation. Conversely, a RAW file is NOT a photo but rather the data required to construct the photo. For example, the Canon software can change certain parameters (sharpness, white balance, tint, etc.) in post-processing that would otherwise have to be re-shot to achieve. Lightroom handles RAW files well but I don't believe it does it as well as the actual Canon software for certain parameters.
Third, there has been much discussion regarding the technology of the full-frame 21MP sensor in the 5D and how photons are recognized/recorded. It seems no matter what lens you use, some softness will occur. Remember, however, that the brighter a subject is lit, the better the clarity will be and chiefly due to more photons hitting the sensor. Also note that the sharpest results are realized with a prime lens rather than a telephoto or zoom lens of any sort. Even so, the lens I use most is the EF 70-200mm f/2.8L IS USM. For the shot you present, I might well have used my EF 24-70mm f/2.8L USM.
Fourth, your full-size photo wouldn't normally be posted on the web "as is"; it would be downsized to accomodate whatever site (say Facebook, for instance) and therefore gain "sharpness" in the process. If you print the photo, DOF will not likely be noticed below 8x10. That's something to keep in mind when shooting.
That's my two cents.