I'll touch on the humidity a bit.
This is actually a big issue for planes; they tried to make it a bit better on the Dreamliner and were able to bump it up a bit
Humidity is improved, yet still dry as the desert. Humidity levels on the Dreamliner are 10%-15% — better than 7% on other planes on long trips
but it's still a big issue. There are two main reasons they can't really increase it that much.
First off, water is very heavy. Since planes pump in dry exterior air they would have to humidify it which means they need to carry enough water to do so. This weight would greatly affect the useful load of the plane. Now you could try to recycle some of the ambient moisture, but on a large scale that takes power, which takes fuel, which comes back to the weight issue.
The other issue (although not as often thought about) is the longevity of the airframe. The extremely dry conditions that airliners see for most of their time actually helps to prevent corrosion on the airframe as well as rust on any rustable parts. By pumping moisture into the airframe you will run the risk of it causing real damage to the metal components.
The temperature is a simple energy problem. To keep a plane hot you need to heat it faster than the exterior temp is cooling it (or evenly to keep it a constant temp). Insulation can help with this but you are still going to see some cooling. On a rudimentary level you can apply Newton's law of cooling here and keep in mind the exterior temperature is in the -40 (and below) area (according to today's charts).
On to the pressure,
This, of course, is a big one, but, in reality, it's not a big deal. While it may seem a bit irritating, 8000ft atmosphere equivalent is still more than breathable. The FAA does not require continuous O2 until 14,000 ft for unpressurized planes, so an 8000 ft equivalent cabin is more than fine. Now from a comfort standpoint, your ears may pop and you may feel some discomfort, but it's more than safe.
From a purely technological standpoint we can do this, but from an engineering use case standpoint it's better to sacrifice some of the pressure to build a lighter, thinner plane. This saves on fuel and material costs down the line. There is great hope that carbon fiber may bring a change to all this and 8000 ft equivalency will be a thing of the past. Boeing has been pushing this with the 787 having only a 6000ft pressure level which they claim alleviates problems. I have not yet flown one so I don't know first hand how much better it really is. Then again I fly unpressurized stuff often and have become somewhat acclimated to it.