Many metal to metal seals have a ball and seat profile/interaction whereas they “coin” when made up tight …Theory, the pressure in the line actually tightens the seal.
I expect the line expands a bit under pressure and that presses the flare harder into the flare nut and fitting.
But that's just my theory. Maybe someone who designs hydraulic systems can confirm or deny.
Those are large items that are prone to warping or poor machining tolerances. At least with brake lines they are sort of malleable and mesh together to form a tight seal.
JIC 37degree. On our power plant Electro Hydraulic Control Unit that supplied oil pressure for main steam turbine control and stop valves, all the connection were Parker Four Bolt flange connection that used an O ring. 3000 psig.Lots of high pressure hydraulic hose fittings are metal to metal.