To them it isn't "legacy code" but just "code", while attempting to offer alternatives either earns you blank stares or expressing concerns about anything new causing inconsistencies with the old (which is a valid concern but also doesn't help when the supposedly consistent code is unusable).
To me it feels like it's also a social problem, not just a technical one and if your hands are essentially tied im that situation and you fail to get the other devs and managers onboard, then you'll simply have to either be very patient or let your legs do the work instead.