How can an annual that is so favored by deer ever become so widespread? i dont recsll ever seeing this flower.

It is perplexing :-) I also have it popping up in various areas of my garden although I am diligent about removing whenever I encounter it and have rarely ever seen it flower. I assume that when it does flower it can produce a large and long lasting seed bank.

btw, deer in my neighborhood never seem to bother this plant. Apparently everything else I grow is much more appealing :-)

kitasei2, some weeds become "adapted" to being browsed and tolerate by ever-persistent sprouting. Ex, you can pull up all the green parts of dandelions & they just keep coming back relentlessly.

The mule deer here sniff around to find what they like - last winter I actually saw a couple of them walking through a treed area during a snow period and swinging their heads back and forth in a scanning movement. So if yours like your pachysandra the fact that nipplewort is growing with it isn't going to prompt them to come more often to the pachysandra - here they wade into dense mounds of trailing blackberry, twiggy clumps of snowberry and so on to get at the new growth of recently established, few caned Himalayan blackberry plants. (Which of course is actually a useful behavior because Himalayan blackberry is a major nuisance plant in my region).

Pachysandra is reliably deer proof here so it was strange to see it browsed. i think it was collateral damage from thrm gobbling the nipplewort which thry clearly love.

How can an annual that is so favored by deer ever become so widespread?

According to the info on the Net Lapsana communis is an annual or short lived perennial.