Old films were shot in celluloid, no pixel count, better than HD…..
Yes true. But a lot of 'HD' transfers to blu-ray etc. are being done from crappy old prints rather than going back cleaning up the neg and then digitising it. Costs more that way of course.
Lots of films are still shot of film and then digitised for the edit, then put back on film for projection. The digital master can then be used for blu-ray/broadcast compression etc. Didn't used to be an option so many old films only exist as old prints, of varying quality.
Another element in all of this is how the material is compressed and sent to you. Different channels and different broadcasters/providers use different amounts of compression and many have 'HD' channels that are actually only broadcasting in 720p which your digibox/TV/Amp or whatever then scales up to 1080p. All of those hurt the way it looks too.
Of course all this is irrelevant to the fact that if they make bazookas out of jubilee clips, it might just look crap however you watch it.