Because having a clarifier adjust both RX and TX is dumb. You only want it to adjust RX only. Think about it, what happens if it does both?
The odds of running into more than one person with a locked clarifier are close to none in my area (has been for decades). Once in awhile, when DX is rolling, I'll catch a station enough off center frequency to be considered locked (along with the many attempts by other stations to get him centered) but once again, it's very rare (in my experience)