Why would you think it's harder out of curiosity?
In the case of SpaceX, consider their last launch failure. The general consensus seemed to be "well, rockets are really hard. And it was the strut manufacturer's fault to some extent. They'll do better next time, and they're still changing the world. Anyway, SpaceX rockets are 4x cheaper than any competitor so even with the occasional launch failure it's still the best option!"
Can you imagine the reaction today if e.g. a Boeing plane blew up on launch? In fact, recall all the criticism Boeing received after issues with the 787's batteries...
But again, I certifiably don't really know. Someone could write about energy requirements and wet/dry mass and how thin rocket skins are and aerodynamic forces and easily convince me rockets are harder.
Think how many more cycles a plane goes through a day / month / year / decade. Times that by the number of models of that plane out there.
Building a totally new passenger airplane just seems a hugely complex thing. Getting it to fly is one thing but you need safety, long-term maintenance, fuel efficiency, international regulation and many other factors.
At least from the outside a rocket looks simpler to me.
Don't get me wrong. I would love to see them succeed but it seems really, really, really hard. Much harder than anything any of the current unicorns has achieved.
From a technical POV, I don't think this is anywhere near unicorn territory. In fact, I think they are aiming too low. I'd ideally want a supersonic plane that is quiet enough to fly over land and cheap enough that your average traveler can afford if they want to trade price for speed. Full autonomous capability so that the plane itself can have thousands of flight hours under its belt before a human test pilot gets on-board.
Yes. Also, regardless of the human pilot, this is intended as a passenger aircraft. That necessitates people on board for it to operate as intended.
Boom can't do this. Unlike SpaceX, they need enough investment money to take them all the way to passenger-safe.
Also passenger safe for a few highly trained and risk aware astronauts is a lower bar than passenger safe for the general public.
The military has used supersonic aircraft on cargo missions in exceptional cases, actually. Their supersonic aircraft tend to be vastly less efficient than even Concorde, though.
To answer that
https://en.wikipedia.org/wiki/Tupolev_Tu-160
and
https://en.wikipedia.org/wiki/Rockwell_B-1_Lancer
The only long range strategic bomber developed after the 50s that wasn't supersonic was the B-2 stealth bomber (not that there have been very many strategic bombers developed after the 50s).