Would be boring if everyone had the same abilities.
There is this thing called diversity which makes something still fun to watch after 20 years of serialization. Free haki hand outs don't fit into that picture. Besides now we can see the growth of the SH's in the New World and not have it off paneled and give everybody cheap upgrades.
Usopp for example developed his CoO when he needed it most to the point he can snipe people he can't even see. The way that was introduced in the manga was different from Luffy having his CoO where he mostly uses it to dodge things. Usopp also shot seastone handcuffs around Ceasar Clown's body to capture him to overcome his weakness in CoA. I much rather see them be creative then doing the same trick over and over again.
Besides they are the main characters, little spoiler on my part, but they will probably make it to the end.