I suspect they do not want to map their 'WE HAVE CHOSEN' keys/mouse use to the system settings or adapt 'help' instructions in game to the actual keys users are using, TBH! Yet again an indication of poor, lazy,overworked,under supervised or arrogant programmers.
(ie - Thief 1,2 are fine but T3 is a POS about this) Now your primary select button is your right button, and the secondary button (commonly referred to as right-click) is the left button. Check the Switch primary and secondary buttons box to make your mouse more left-hand friendly. Only the obstinate apps do not use system settings anyhow. Enter mouse in your Start menu search, and select the first entry. I bet it is very simple to use the system settings but poor or insensitive game programmers ignore doing it. I suspect, but do not know, that it would not work simply because the game must 'honor' your settings versus ignoring them as many games do. I think quite a few mouse-vendors allow custom mouse-button assignments using the Control-Panel->Mouse Settings Panel, which would then work the same for every program you use, but i guess you looked into that already.