Why didn't they? Couldn't a team of Spartan-II's escorting some naval personnel board a Covie Battlecruiser and capture it then allow the naval personnel to take control of it? Rinse and repeat.
I mean we all know that UNSC ships are garbage when facing Covenant ships. So they should've just captured Covie ships to level the playing field.
Would've been pretty effective in my opinion.
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They tried it. The Covenant always self destructed.