The thing is, it's not as easy to avoid lines as druidh makes out.
I live in Sheffield and on my old commute you HAD to cross the lines twice. No way around it, at all- if you wanted to cross one street you would have to take a 7 mile detour. This is after having ridden in a 2 foot wide gap with traffic and trams squeezing by.
I have crashed on the lines that you have to cross, I hit a car and caused damage and cracked my frame, bent my bars, trashed an STi and bent my rear wheel. I'm an experienced rider and had slowed down significantly for them but the angle you hit them on a wet day means that you're risking sliding into the groove regardless of how good you are.
No one is immune from this, but inexperienced riders wobbling all over the edge of the road aren't stupid for falling into them. I'd say 90% of cyclists don't realise that they are at particular risk of sliding on metal. It sounds stupid to you, but it's not obvious. A drain cover poses no threat at all when you're walking or in a car and if you've been cycling 6 months with no cycling mates to explain things to you then you aren't going to leap to that conclusion.