Teach is looking for DF masters since DF masters are the strongest characters in the manga, hence why Teach is looking for DFs instead of just training Haki. They could’ve easily spent 2 years developing strong Haki, Luffy learned advanced Haki in like, 30 seconds every time.
He learned advanced armament Haki in 20 seconds at Udon,
He learned FS in 11 hours vs Katakuri,
And he learned AdCoC exactly 3 seconds after realizing AdCoC existed lol.
Haki is easy to train and master, but finding Devil Fruits (strong ones at least), is extremely difficult. And yet Teach still chooses to hunt for strong fruits instead of taking the easy road and training Haki, because he knows that taking the harder road will pay off more in the end.
Haki is for failures like Kaido, Devil Fruits are for true god tiers like EOS Teach and the Admirals.