When a word ends in two Seggols (or a Seggol and Patach, depending on whether the final letter is guttural), and the word is in pausal form, the first Seggol is lengthened to a Kamatz. Why is it not lengthened to a Tzereh?

Is it due to historical changes in our pronunciation? If so, should we adapt the pausal form of these words to better reflect our current pronunciation of the non-pausal form?

(I am aware that there are exceptional words which don’t have a pausal form. They do not pertain to the question.)