I got my character created, my laser pistol in my holster...now what? Do I have a job at a starport? How much does that pay per day? Are there mission, planet, and star system generators? Can I easily create alien species, robots, starships, security systems, and NPCs? Are there mission creation systems?
Sci-fi is a lot more complicated than fantasy. You can run dry on ideas in an instant. The nature of a dynamic environment in three dimensions, with cultures, cities, space travel, and billions of factors, can overwhelm any game master.
Every tool a game gives you goes a long way.
Frontier Space has a lot more of that low-level sandbox sci-fi information. Where a game like Stars Without Number is incredible for creating planets and fantastic adventures, the nitty-gritty for how you do a 'street level' sci-fi campaign is fully realized in FS.
Where 5E games often fall short is in the limited space and energy they have left to do 'the other stuff.' You will have unique characters and powers, but very few 5E games cover the rest of what you need. This has been a curse on 5E ever since the original 5E DMG was released, and the world took that as, 'Oh, game mastering advice is not important.'
I can find plenty to do for my Frontier Space pilot-engineer. There is a table for wages and pay grades. If I have a million credits, I can buy a scout ship.
My Esper Genesis character awaits a published adventure and boxes of credits to open in space dungeons. The game does an incredible job with classes and powers, but like 5E, what lies beyond that is weaker and depends on published content.
This is the same problem Starfinder had for me. The entire starship economy in that game is lost in space, and taking cargo for profit to buy ship upgrades is hand-waved away. They have this strange space socialist society where starships are given to parties, and the upgrades are free. I still felt there was no sandbox to play in, nothing to the world on a low level, and the game revolved around the adventure path. All the flashy powers in EG or Starfinder are useless without supporting material.
Again, another difference between old-school games and new. OSR games give you the tools to fuel your imagination. 5E and -Finder games are tools to sell published adventures and put your wallet on a content stream.
Give me Frontier Space, Traveller, Stars Without Number, and any other classic or old-school sci-fi game, and I am set for life. I like the Esper Genesis vibe, and it feels like Mass Effect.
But I can get space magic through modding Frontier Space with Barebones Adventures. I can also get trading and exploration games with Traveller or SWN.
Many of these 5E games are fantastic, but many are just flashy collections of classes and powers and not much else. I see many Kickstarter 5E games, too, and most of them follow that same model - lots of cool character options, and behind that façade, the support for living in a world like that feels lacking.
5E is fading fast for me, outside of Level Up Advanced 5E. That game backs up the other pillars of play, and every other game - even 5E sci-fi - feels lacking compared to the entire sandbox and social play support that A5E gives me. A5E spoils me regarding 5E, and the other wannabe 5E games feel like they have pieces missing.
Even stock 5E feels weak compared to the better character-building systems of games like Shadow of the Demon Lord (and the Shadow of the Weird Wizard Kickstarter, which I am in on).