Mainly because even if it does boost itself, Charge Beam does less damage than Thunderbolt. Also, with all the switching in competitive battling, you're rarely going to hang on to a boost that long. Futhermore, Charge Beam has a 10% chance of missing. Thunderbolt is 100% accurate and is much stronger, Discharge is somewhat stronger and is 100% accurate, as well as having a 30% paralysis chance, and finally Volt Switch is slightly stronger, and allows you to gain switch priority.
Tl;DR: Thunderbolt, Discharge and Volt Switch are more accurate, stronger alternatives with better effects, or they do so much more damage that the +1 doesn't matter.