You`ve got a point there and usually bookmakers landing pages are about languages offered etc. 
I think they don`t need them or don`t like them otherwise there would be plenty.
Concerning Gamebookers: they didn`t have a landing page for years only recently they implemented them (I know of two). The free World Cup shirt offer made it to their main page only week or two ago and I found it odd too but as Gooner said there are special links available pointing to that particular offer landing page. So, it takes a bit of work but you got to put some effort to get good results.
cheers