For instance, the Canon XF405 camcorder has an 8.2 megapixel sensor, yet it costs over $3000 USD. Whereas even a iPhone 6s has a 12 megapixel sensor.
Why do dedicated video camera have sensors with lower resolution than even camera phones?
To expand on @twalberg's comment: you don't need more than 4K, and professionals don't want to use something bigger that the strict necessary. All the extra pixels cost money: storage, processing time.... I worked for a TV channel a couple of years ago, and a mix of 720/1080 programs (we were not even talking about 4K...) was already 3 petabytes[*] per year (double if you count the backups)(broadcast-grade video was around 60GB/hour). They also run specialized computing farms for transcoding, automated quality checks... All this infrastructure would be at least 50% more expensive if you want to support 12Mpix.
In addition, the 8Mpix of that camcorder are likely to produce better images natively than the 12Mpix of the smartphone with state-of-the-art post-processing.
[*] More or less you living-room wall lined with 2.5" 1TB units, small edge along the wall.