I think you missed the point if this tool Alectric. While you could hypothetically load every browser onto your computer that would be a lot of browsers. Additionally many browsers do not let you keep older; working versions of themselves on the same computer (for example, having Internet Explorer versions 6. 7 and the most current version, 8, would be a tough thing to make happen).
Also, even if you were able to load all those many different browsers and versions onto one computer you would still have an issue with operating systems. Browsers behave differently, and have different versions for different operating systems. So having a computer with Windows XP Home, XP Pro, Vista, Windows 7, and for all I know there may be differences based upon what service pack the OS is working on. There were three different service packs for XP. All of that is just for Windows PC. Then you would either need a Mac or some hybrid that lets you run Mac operating systems because browsers operate differently on Mac's and there are several versions of the Mac operating system.
And don't even get me started on Linux. Soooo many versions.
Then there are the resolutions. Some resolutions are impossible without the right size monitor and even with the right size monitor the right size resolution might be impossible due to software incompatibilities or hardware limitations.
And even if you could do a lot of the resolutions yourself constantly changing your resolution would be a royal pain in the ass.
Sites like browsershots and browserseal let give you screen shots of many different browsers on different operating systems at many different resolutions all at once. This is an important tool for the web developer who is trying to make whatever site he is working on compatible with lot's of different computers.
So while the browsers may largely be free just having most or even all of the browsers on one computer is only the start of what you would need to full test compatibility.
I hope that explains it.