No one ever said Hollywood was original.
You have to remember what Hollywood is. It is a business. More importantly, it is a risk averse business.
Hollywood is an industry built on reputation. "You're only as good as your last film". Ergo, with the stakes being so high... Millions of dollars per film... The people who actually drive this industry... The studio execs and not the artistic persons like directors, screenwriters, actors etc... Are going to want to make sure they are going to get a sizeable return on their (the studio and investor) investment. That's just good business sense. Forget artistic sensibilities.
This is why Hollywood is obsessed with remakes and comic book movies. They are guranteed hits, more or less. Especially, the remake since it supposedly did well when it was released originally, ergo, an updated version should do just as well according to their logic.
And like you, I am not a purist in I think some remakes are better than the originals... But that may be because of my/our contemporary culture bias I/we have just from being born when we were born as well.
Also, the other big factor is that studio execs think in terms of marketing to one demographic and one demopgrahic only: 14 year old males.
This is another reason comic book movies are so popular (even if there are good ones out there like X-Men and Spider Man).
This is the core of how marketing decisions are made, what films are going to be greenlighted, who is going to be cast in the film, etc, etc. Hollywood loves to claim that they want a wide audience... Which is true in some respects... But deep down, they cater to the 14 year old male with a lot of disposable income who cannot only go see the movie, but buy the video game, the soundtrack, buy the DVD once it is out and other tie-ins moreso than the 25-45+ year olds who just see a movie once and that's it.
The Birds fits into this category because they can potentially knock the rating down to a PG-13 "for intense animal violence" (or something) and get all these teen boys into it who've never even seen the original and have no desire to. Cast someone like Jessica Alba, or Jessica Biel in the Tippi Hedron role and its instant teenage boy appeal.
Face it: Hollywood is about making money and catering to the youth market.
While this isn't new... It has never been to the point where it is now where adults (25+) just do not factor into the movies being made and this is partly why there is a lot of crap out there that doesn't appeal to adults and why a lot of adults don't go to the movies (coupled with the high price of tickets and concessions) anymore and why downloading films off the net is taking place of the theater going experience for a lot of movie fans.