To expand a bit more, having done some more reading/thinking :22:.
Regarding environmental stuff, it's some chemicals and compounds involved in the making, and I think also solvents used to clean parts before final assembly. To expand on this, existing tube manufacturers have made changes to their products in response to the wide-ranging RoHS directives first implemented in 2006, as one typical component of tube manufacture is lead oxide (PbO, which, when added to glass, increases its electrical resistivity, hence its use in tubes). As applied to tubes, the RoHS ruling was about restricting the threshold of how much of the stuff could be in the glass by weight. As with most types of regulations, they are constantly reviewed and amended, so it could be different today compared to 15 years ago.
Of course, back in the day, environmental issues weren't so much a consideration for the reason tube manufacture dwindled to begin with - the world of electronics simply moved on. Transistors with semi-conductors are much easier to manufacture en masse and through automation.
Another factor is the special machines (e.g. for creating the vacuum) and tooling (for grids) required to make tubes. When everyone was moving to transistors, they were scrapping this stuff, so unless you can find an original and refurbish it, making new machines and tooling is mega bucks, even before you make the first bottle.
Old tubes are good because they were made to exacting tolerances and they have considerable longevity and an ability to take a beating. How much they sound better than new ones is more subjective. Different, surely, as, like with guitar pickups, the alloys used in the metal parts were different then and today.