You've Been Boiling Your Vegetables Wrong And Pouring The Goodness Down The Sink
The nutrients are leaving with the water.
You boil your veg until soft, drain them, and tip a pan of grey water down the drain. That water is not just water, and getting into this habit means you have been throwing out part of the meal.
Many vitamins in vegetables are water soluble, which means the longer they sit in boiling water, the more of them leach out and dissolve away. Overboil and you are left with softer, paler, less nourishing veg.
Steaming is your friend here. It cooks vegetables over the water rather than in it, so far fewer nutrients escape. If you do boil, use less water, keep the time short, and stop while there is still a bit of bite.
You can even reuse that cooking water as a base for soups and gravies. Cook veg quickly and gently and you keep both the colour and the goodness on the plate.