It's The Climate That Does It

That, and the infernal distances.

Space is big.

One can harp on the topic endlessly and still not impact any understanding on someone.

It's mind boggling.

For example:

Let's make a scale model of the Milky Way.  The Sun will be the size of a grain of sand in our model.

How big is our model?

Hope you have a big back yard, because at this scale the Milky Way is the size of our Solar System!

Distance starts having a massive time component to it.

It's about eight minutes to The Sun from here at the speed of light (1 AU ≈ 8 light minutes).  Jupiter is 36 to 52 light minutes from Earth (depending on the time of their years) one way.  Conversations are kind of hard at this scale.

Atomic Rockets makes a lot of salient points about detection.  It's damn near impossible to hide a manned ship in a solar system.  They're just too damn bright-hot in several easily seen frequencies.  Where Atomic Rockets goes wrong is forgetting resolution.  The longer the wavelength, the larger your aperture needs to be for a given resolution.  A comfortable 24˚ C is very bright in infrared against the backdrop of space, but you need a really big aperture to see more than a dot.  Knowing it's there from the dot is one thing and the temperature range tells us it's likely manned.  Is that enough to start shooting at it?

Atomic Rockets does a good job of explaining that you can detect a manned ship out at Jupiter's orbit readily.  Remember that time component?  You're detecting something 36 to 52 minutes ago.  That's a long time.

A Traveller missile is only good for 50,000 miles from the launcher before it loses its command guidance.  This distance is mostly dictated by the time delays of the laser commanding the missile and how stale the location information on the target is, a 1/4 second distance, which is a 1/2 second stale at maximum range which is actually a lot of delay.  An M6 ship can be more than 7m from where you think it is in that time.  That can be the margin of missing.

Of note is a standard TL10 missile pulls 6g for an hour.  That's a "mere" 473,997.7 mph and the thing weighs 300 lb. (convert that to mass how you will).  However, it will have travelled 236,998.9 miles (almost to the moon, Alice!) (1.27 light-seconds) in that hour.  Based on the limitations of the laser command guidance, much of the delta-v of the missile must be for lateral movement to intercept the target rather than to merely build speed.

An autonomous missile could go a lot farther and its targeting delay gets shorter not longer as it approaches the target.  The trick is the terminal stage since to get to any appreciable range in a timely manner you need a lot velocity (GURPS TL12 Traveller missiles have 55g! for a short spurt).  More velocity means it's not as easy to make corrections and the staler your information is the farther off your initial vector can be.

Something that Atomic Rockets tends to get wrong is stealth and decoys.  The object is not to escape detection, it's to escape the terminal phase of missile guidance.

Remember that whole aperture size and resolution thing?  The same rules apply to collimating a laser.  The farther you want to shoot, the larger your mirror/lens diameter has to be for a given frequency.  Again, as information about the target's location gets staler, the likelihood you missed goes up.  The shorter the wavelength, the smaller you can make the mirror/lens, GURPS Traveller says they're ultraviolet at TL8+ and X-Ray from TL-10+ with some frequency adjustment becoming available at TL9.  Ranges are modest, a mere 17,000 mile 1/2 damage range for a standard TL10 turreted laser.  These ranges have a lot more to do with how tightly you can focus a beam than how far light will travel.  It's a simple equation that I forgot to write down and can't find the wonderful tutorial I learned on anymore.  Perhaps FuzzyGeff with his tasty chess club brain will be along with it.

The same sort of thing is happening with particle weapons (plasma, fusion, particle beam and meson) too.  Of course the details of how these weapons work is hand-wavium...