Roo makes a good point.
I was at a reptile show on Sunday where a snake keeper was displaying absolutely beautiful wood and glass cages. They were crafted so well, that they were more like exquisite pieces of furniture than reptile cages. He was selling them at the equivalent of about $800 and upwards.
I asked him about modifications to put screen sides in, instead of glass, "You know, for ventilation", I told him.
He said that would be absolutely unnecessary. He showed me the fans and vents that were installed and corresponding gauges and knobs that controlled them. He had an emerald green boa in one cage (I'm not a snake expert, so I can't tell you the species) that he claimed has one of the most demanding environments to simulate. But he said that inside the glass, he could more accuaretly control the temperature, humidity and airflow than in any other enclosure.
Glass CAN be used to keep chameleons, but it requires much more care in setting up than with a screen cage.
And that's why screen cages are recommended. Most new chameleon owners are getting chams at pet stores who do not instruct them in properly setting the cham up.
So it's just easier to recommend to inexperienced keepers to use a screen cage.
As someone who started keeping chameleons with very little experience of other reptiles, I'm glad I went the screen cage route. There are so many other things to worry about when developing chameleon-keeping skills, that I'm glad I didn't have to worry about the glass.
Personally, I also feel that it's easier to keep screen clean and looking good than glass (which shows up every spot of water). And that's important, since chameleons are pets that are observed (not played with), and it's far betting watching them in an attractive cage than an eye-sore.
And then there is the argument about the stress caused by a chameleon's reflection. I'm not sure this one has been proven entirely yet, but since a lot of things about chameleon husbandry are still not completely scientifically studied, it does provide motivation to follow the anecdotal advice.
So as for all those older chams you've seen living in glass houses, either they have very knowledgable and meticulate keepers who have them set up perfectly inside there, or, as Kinyonga always says, those chams are surviving, but not thriving.
. Heres is a pic of how it looks like 
