Question

Hello Rabbi,
The Sages very often use verses for aggadah and Jewish law by changing the simple meaning of a word as it appears in the verse. For example, “Do not read your children, but rather your builders,”
or “Fields shall be bought with money” — the Sages change the pronunciation of “shall be bought” with a pataḥ instead of the ḥiriq that appears in the original. And all this is in order to fit the Jewish law they already have to the verse.
Is there a reasonable explanation for these moves? If not, then why is it needed? Let them just say, for example, that this is the Jewish law, and that’s it.
Thank you!