Ah, the age old "but I'm not touching you" argument. Truly we are learning at the feet of the masters!
With respect, I want to point out that these factors are not really relevant, in my opinion. You can do these things and also include ways that don't make magic the only path forward.
This all sounds like a lot of fun.I've had a lich with a hidden strongbox type place accessible only by teleportation and known to nobody else in the world. A dragon nursery & day care in what was effectively a magma submarine. A wind-mage's home flying in a permanent hurricane. A demon lord using tarrasques as ammo. An invisible brick wall in front of the landing spot on the other side of the barely jumpable chasm. Anything resembling normal PCs simply can't deal with some stuff without certain abilities that noncaster classes don't have.
Of course then there was the not-mount-Olympus where gods literally hung out partying a lot of the time. No form of magic would get anyone to the top, and no form of anything would save them from the consequences. But hey, a high level fighter could at least try to get noticed. Plane shifting was much safer, you show up at the front door and the flunkies kick you out instead of popping onto the dance floor and getting stepped on. Weirdly, magic also didn't help much if you wanted to join the fire breathing arctic dire bear calvary.
And I suspect that your players enjoy it as much as you do. And I think this is reinforcing behavior. Players might know when they sit at your table that they're going to need spells in order to interact with the things you have in mind. And it just becomes self-fulfilling.
But I wouldn't make the mistake of thinking all games are like this or have to be like this. Or that these same encounters/scenarios can't exist in a different game without spellcasters, and they have to use other means to engage.
There are random encounter tables for all levels of play though, and a DM can also just put whatever they want during travel.
It's really just up to the DM what they find interesting or appropriate. We can't say for all games.
Agreed.
I mean, a lot of this is just preference or style related.
Take the utterly ridiculous claim made recently that a campaign around a group of all fighters traveling the planes sucks and is bad writing. This is preposterous. There are magic items that let you travel through the planes. Have we never seen stories about a guy or a group of people using a device to travel to other dimensions? A campaign of martials plane-hopping could be a blast to play and run.
Unless you have very specific and narrow ideas about how people should play D&D, and insist that if people are going to plane-hop, it MUST be through the Plane Shift spell.