Someone tell me why I cannot find the simplest weedy plants, merely because I want specific plants?

The day before yesterday, I needed yarrow, a very ubiquitous plant...and I could not find it anywhere. Well, actually, half of the problem was that I couldn't find any with seed heads, and the leaves are difficult to distinguish from other apiaceae plants such as Queen Anne's lace and various lomatium species. Some of it may have been yarrow, but all of the seed heads were something else and none of the leaves were unequivocally yarrow so I played it safe, because it was going to be used as an herb. And arnica- forget it! I didn't expect to find arnica that day anyway, it's not as easy to find as yarrow. Yesterday I needed mullein. Mullein can be seen by roadsides, in fields, everywhere. It's that plant with the very soft, pale gray green leaves arranged in a ground hugging rosette, which sends up a tall flower spike in its second year, sort of phallic looking with little yellow flowers. I couldn't find that either. Maybe this is because it's getting late in the fall....I need to pick a bunch of it when I find some, it's good used as a tea to relieve sinus congestion from colds.