I think that's probably because bicycle lights fidget about with the riders movements, so the glare fluctuates, also of course, they don't have dip beam!
For the benefit of American, and other countries that don't realise, daylight running lights became mandatory in the UK from 2011. I think, but could be wrong on this, some manufacturers just removed the headlight switch, so the headlights became the DRL's, others fitted, or had already fitted dedicated DRL's.
This was possibly an EU influenced law!!
Motorcycles are exempt, but a bike I bought new in 2018 has a permanent on headlight anyway! I don't like this because it's a nuisance when you are working on the bike in the garage, especially at night, and the bl00dy headlamp is on!!