No bullet, but from my perspective I think that it's because CA is a fairly loose arrangement (and all the better for it in many ways) and the key to the question for me lies in the CA constitution:
"The purpose of Club Arnage is to provide a web-based forum, that is renowned for friendship, openness, humour, honest information and the absence of advertising and commercial gain, as a home for any individuals who share the many and various dreams and interests that make up the Le Mans 24-hour race, and much else that takes their fancy besides".
If there were someone with the time and inclination to make the necessary arrangements and administer a system, it's not that we couldn't use an agency, or even offer a single collective solution, but that with such a wide variety of individuals and groups, who all have different foibles, and a mixture of hardy regulars and others who only use the forum around race time, it is perhaps inevitable that people make their own arrangements to suit their style of enjoying the event, whether that is through being on a particular campsite, having a distinctive encampment, or going as a group or as an individual. Consequently, we have the panoply of talent that makes up CA, with some using existing agencies, others booking direct with the ACO, and others taking their chances nearer the race weekend.
MG Mark