Terror of the Autons hasn't been released because it only exists in black-and-white and needs too much restoration work. As for
Planet of the Spiders, well - I'm right there with you!
Correction: Terror exists in colour, but in the same reconstructed-from-two-sources form as Silurians (which has been released) and Daemons and Ambassadors of Death (which haven't).
So it could be released, but yes, it ideally needs work, as the current reconstruction was done 16 years ago, and a far better result could be produced now by redoing it, either by the improved version of the same method (as on Silurians), or possibly by using the new colour-recovery technique used on Planet of the Daleks 3 (possibly as the B&W film prints can't always be used for this: if the technician making the B&W copy did his job properly and put in the widget to remove the colour information, then there's nothing there to recover).