As long as it isn't too disadvantageous for female giraffes it highly likely that it is easier for evolution to end up with both sexes having long necks, for th

As long as it isn't too disadvantageous for female giraffes it highly likely that it is easier for evolution to end up with both sexes having long necks, for the same reason male mammals have nipples. Sometimes the genes involved are so disconnected from sex-specific genes or the body plan/development process such that it would require very high selective pressure to make it a dimorphic trait. Or those tissues just don't have sex hormone receptors so they don't respond to them. Or it just so happens that certain structures are built in an earlier phase of development before sex hormones can be produced. Evolution is not directed; it is far easier to tinker with what already exists than make wholesale changes. Selective pressure for nipples exists but for whatever reason it was easier for evolution to just give all bodies nipples even though they don't do anything in males. The metabolic cost is a rounding error so there's little pressure to get rid of them. If I had to guess the hox genes or whichever genes that encode the repeat division sequences to make the neck bones be any size at all have no pathway to differentiate by hormones because it happens far too early... so it is simpler to have all giraffe body plans build the same basic neck structure in the fetal stage. If you compare to humans boys and girls are born with the same hip structure and probably for the same reason - it is only during puberty that they diverge. And even then they don't undergo a complete remodeling.