While it’s a very long story, the short answer is that technology in the 41st millenium has regressed to a point where people no longer truly understand it, meaning that the simple act of turning on a cogitator (40ks equivalent of a computer) is a ritualized undertaking involving candles, incense and prayers to the machine spirit of said cogitator, which will, during some point of it, involve pressing a button a very specific way. The fact that to turn on the cogitator actually only requires pressing that button is unknown to humanity.
In fact, most humans don’t even have that amount of understanding and have to rely on people like Hadron (so called Tech Priests - the clue is in the name) to maintain their equipment for them. Modifying or wrongly treating technology is considered so called tech-heresy, which is the reason why there is next to no technological progress in the Imperium. In the end, it would probably be possible to attach a sight to a gun, but it’s nothing that is easily done and would likely require the tech priest to conduct a ritual to actually make sure the machine spirit of neither the gun nor the sight is offended.
No, I’m not kidding.
Oh, as for Melee: The advent of effective body armor that protects well against most infantry-sized guns actually made Melee a significant part of 40k. Flak Armor for the imperial guard is capable of turning a lasbolt from a regular infantry-sized lasgun into a hurting bruise on the longest engagement ranges, while better armor (Carapace armor for elite soldiers or possibly even Power Armor for some very elite groups) shrugs off even massed lasfire (or autogun fire, even though autoguns are more of a backstop measure compared to the lasgun). Add to that the fact that most enemies of the imperium tend to have significant counters to ranged weapons (armor for instance, supernatural speed for others, holofields that make them basically invisible or, in the case of the Nurgle-aligned chaos forces we’re fighting in Darktide, just superhuman amounts of toughness because of Warp (read: Magic) fudgery, and suddenly a chainsword becomes a very coveted weapon.
Also: Don’t think that melee is completely absent from modern battlefields either. Trenches, ambushes, and urban warfare make melee something one should never underestimate. There’s a reason soldiers still carry a sidearm and/or a knife or bayonet with them.
HOWEVER
I completely agree weapons should be customizeable. I even started a post about it here: