Two factors:

1) There are a lot of places with Islamist violence that are not safe or in some cases simply not permitted. This is much worse than it was in the 70s.

2) You don't get a free choice of routes. There are a limited number of countries and going around isn't always an option.

Let's consider the two routes I did part of in the 70s/80s:

Katmandu to London. (Note that I'm not 100% sure I'm remembering the route right. I'm trying to reconstruct what I can remember with a map.)

Katmandu to Delhi: I wasn't on it for this section, I do not believe it's too problematic.

Delhi to Lahore: Pakistan??

to Islamabad: Pakistan????

to Kabul: Nope!

to northern Afghanistan: Nope!

to Karachi: Afghanistan, Nope! Pakistan????

to Shiraz: Pakistan???? Iran, Nope! and denied.

to Tehran: Iran, Nope! and denied.

We did not plan on staying with the bus past Tehran so I'm not confident beyond that, I think it was going to go to Tabriz, then Istanbul and up through Europe.

Let's see what we can do now:

Google will not map the route. However, I can get a partial map: It is willing to route to Kashgar. It goes *east*. Then south through Myanmar (pretty much a nope). I'm completely unable to get it to go across the India/China border, I don't know if this is political or a matter of roads, I suspect the latter. After Myanmar it goes through Thailand (AFIAK fine), Laos (no idea), Vietnam (I think ok) and up into China. Note that Kashgar is in Xinjang province--I consider that a nope.

Google now fails me. At this point there's a pinch between Russia to the north and Iran to the south. The only route west is via Turkmenistan or Kazakhstan, to reach the Caspian sea. At least in the past you could take a ferry but I have no idea of what it connected. The only viable spot I see on the west coast is Baku, Azerbaijan, then I can force Google to take the route through Armenia and Turkiye. AFIAK Turkmenistan and Kazakhstan are iffy, don't know about Azerbaijan and Armenia. I also haven't paid attention to where you might want to avoid in Turkiye.

Now, let's consider the other long trip I've done. Johannesburg to London.

South Africa: Parts are ok but there's no way I would overland. I can force it to approximate the route we were taking but there's a wall of Congo/South Sudan/Ethiopia that are Nope! places. And then there's a second barrier posed by the Sahara. You can take the western route through Nigeria (Nope!), Niger (no idea) and Algeria (my impression is Nope!) but AFIAK this route is actually forbidden at Timbuktu. Google will also map a route through Sudan (right through the combat area) up to Egypt--but AFIAK there's no way to proceed past that point. You have to cross into Israel (AFIAK temporarily blocked), then either up through Lebanon (Nope!) and Syria (Nope! and you will be denied) or through Jordan, then Syria (Nope! and you will be denied) or Iraq (Nope!, I suspect you will be denied.) Syria/Iraq make another uncrossable wall and you will have two damning stamps in your passport. (Israel will not stamp your passport, but Egypt, Jordan and Lebanon will. Once you've crossed an Israeli land border your passport is tainted and can't be used in a bunch of Muslim nations.) Trying to hug the western coast you'll hit Western Sahara (a Nope!) and I think some of those places along the coast are also Nope!s.