Familiarity I guess. Mega isn't really a widely used prefix outside of computers. We even say tons instead of megagrams.

Megameters sounds scary and I don't want to alarm the people I'm talking to.

I do. Unfortunately, I don't have many opportunities to do so. Which may be the reason why people don't say megameter.

On Earth it’s just not needed. In nearby space it could make sense — distance to the Moon is 369 Mm. Distance to the Sun 149 Gm. But people aren’t good at visualizing the difference between kilo-, mega-, and giga-. It isn’t obvious from those numbers just how much further away the Sun is.

That unit is used a lot in the space game Elite Dangerous. Never saw it used before that, but it made sense because it's the next jump up in large units, and it also helps keep the UI clean looking.

Jokes on you elite dangerous uses Mm/s for lowest speeds in supercruise before it changes to "c" for relative to light speed

As a KSP player I use megameters all the time lol.

nobody will stop you, i've seen some publications use gigagrams instead of thousands of tons

I wish we would. It sounds awesome to say.

Well mainly because where we might need to use these units, we have more standard non si units, we use AU, Light years and Parsecs where Megameters, Gm, TM etc would be useful

Aside of kilometers there used to be "myriameter" (a myriad meters = 10,000 m = 10 km).

Fun thing, in Sweden they use mil for 10 km. In Finland there's peninkulma for 10 km, but it's very archaic.

We already have this in Sweden. 10km in Sweden is 1 mil (Swedish mile).

When we sell/buy used cars and other types of vehicles we always count the mileage in Swedish miles.

Kilometers work but is just absurd when you start talking about 100k+ kms.

Scientific notation for everything: 5 x 10^6 m. Seriously though, I think it would be easier to think about it in megameters or gigameters if it were more standard to do so.

Main reason is nearly no one needs to measure things in megameters. Megameters would be a unit to measure the diameter of planets in, maybe the orbital altitudes of some moons. Our moon for example is ~384Mm away. Distances between planets, distances between stars, and distance between galaxies are many, many orders of magnitude farther than that.

As most of us rarely travel more than 1,000 kilometers very often, it's the biggest unit most people are familiar with on an intuitive level.

I'm still convinced people don't actually use the metric system's power of ten design. Like no one uses centigrams or kiloliters either. They've picked out units that are pretty close to the ones in the Imperial/Customary system, kilograms are used instead of pounds, grams are used instead of ounces, kilometers are used instead of miles, meters are used instead of yards, centimeters are used instead of inches, millimeters are used instead of sixteenths of an inch and so on. Want to confuse a European? Draw up some blueprints in hectometers.

Here in Sweden and Norway we have the Scandinavian mile, "mil", it is 10km.

It is 2.445 kilomiles from Los Angeles to New York.

In our primary schools, we learn our children mili, deci, centi, deca, hecto and kilo, and how to calculate between them.

Beyond that or below that is used either in science classes or specific usecases and not known by the whole population at large.

Since people use what they know, they'd never use mega as a common way of measuring. We mostly use km for distance, and only in specific cases we might use, say, hectometers or decameters.

5 megameter is not wrong, but I don't call 34 cm 3,4 decimeters either(unless decimeters make sense of course :p)

Not much need to use Mm, it doesn't come up very often. So when it does it's easier to use thousands of km so as to not confuse people with "another" measurement.

Yeah but the real question is how many is a Brazilian?