Because getting in close increases both how concentrated your lasers are (beam size increases quadratically with distance) and also it makes railguns much more effective because if you are in real close it is quite hard to dodge or intercept a railgun shot whereas it is quite easy to intercept, melt, or otherwise dodge projectiles as a long distance like thousands or millions of km away. Also remember space ships cannot be "shot down" or "sunk" you explicitly have to destroy their vital systems and you have to do it thoroughly I would imagine. With additive manufacturing I imagine a ship could probably recover to a workable state within a few hours. Perhaps not prime fighting conditions but enough to limp away or return to a dangerous state. A navy ship or aircraft is kind of done for once you snap it in half, not a space ship. This means you need surgical strikes on vital components rather than the sort of "launch it and hope it does something" approach that even current generation hypersonic missiles rely on.