To expand upon what Massive said, your Mackie sounds like it is functioning exactly how it should. The solo buttons should take a channel, and place it in the solo metering path as well as the control room output path but NOT the stereo left and right path. There is however a button on the mackies if I remember right that allows you to mute all other channels in the main path also when you solo things. It is just not a feature which is very wanted in most all scenarios. In fact often times it is very unwanted. Some bigger better consoles make it very hard to activate this feature. I know that at least some of the larger Midas consoles actually have this feature isolated under a plastic cover like a nuclear trigger or something so you can't accidentally enable it.
Consider this, you are running a mix, or even worse, a show or a live mix and you reach out to mute a channel. Instead you accidentally hit the solo switch which happens to be right next to the mute button. Say your current mix was 40+ channels. If you had your solo to mains feature activated you would have just killed 39+ channels and replaced them with 1 channel that you had actually intended to take out of the mix. Also, many solos are often labeled or at least run as "pfl". This means prefader level. What if you had this channel that you accidentally solo'ed down at -40 on the fader to keep it very buried in the mix. By accidentally soloing it you replaced 39+ channels with a channel you went to mute. You also replaced all of those with that bad channel at 40db higher then you had intended. You may have just blown a pair of studio monitors, or a PA rig! Solo to mains is usually not very desirable, hopefully you can see why now