And that is the main difficulty of becoming a successful sports bettor, if you really want to have a chance to become a professional gambler then you have no other option but to watch all the games that you can, and as we know that takes a very long time and only the most dedicated gamblers will invest so much time into that activity, the rest of the gamblers will watch a game or two and nothing more and then they will complain about their bad results.
I don't think you need to do that at all. The internet has all the stats and info you'll need to know to determine which team is bigger chances of winning and which team is the underdog. Of course you'll be more flexible if you know the sport by heart, but anyone can have the same info as someone who does with just a few minute research. The more you try to do that, the more you'll eventually know about the sport you're betting on.
In my limited experience with the few professional sports bettors I have met they can only make profits in the sports in which they are experts, stats are fine and can be a great way to absorb a lot of information about your favourite sport, but if you do not watch it then you cannot give context to those stats and while the advantage that you may get out of this process may seem to be minimal you really need any edge you can get against the casinos to beat them at their own game.