Optimally, a father should circumcise his own sons. Optimally, an aliyah should read the Torah when called up. When we are called up for an aliyah, we say the brocha and the baal koreh reads for us. At a bris, though, the mohel says the brocha. Why doesn't the father say the brocha at the bris, and then the mohel performs the bris for him?
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Your question is really the other way around. Generally speaking it's the person who does the mitzva action that says the bracha (Rambam Brachot 11:10). In the case of an aliyah, you are right that in most congregations an appointed reader is designated to read each persons portion out loud and with proper cantillation; however Shulchan Aruch (OC 141:2) rules that the one receiving the aliyah must read along with the leader in an undertone so that his blessings should not be in vain. We see that even in the case of an aliyah the bracha is said by the performer of the mitzva action itself.