Yes, because customers do not know much/enough about surveillance products to buy the right system for their needs.
To buy the right system, a customer will need to know:
2. sensor size/pixel count/low light picture quality tradeoffs
3. how to run cable legally
4. how to waterproof exterior components
5. al the headbanging quirks and network related intricacies of the how system
The amount of time required to get the above would cost the customer many times more then to just trust an integrator the offers a proper 3 year warranty on parts and services and handle all matters from start to finish. its almost like purchasing insurance - you must have it.
To me integrators are a type of engineer - they optimize systems based on theory and experience to achieve the best outcome with whats available.
The IT industry is actually the same - except that at least in Australia - the consumer/SMB IT industry has been almost completely destroyed by careless cross-selling to "box-shifter" resellers with no knowledge whatsoever. Every week I get called over to a friend's place to fix up their brand new notebook because its is packed full of bloated adware, or a brand new router the crashes once a day, or a NAS that does not work with Windows Backup dues to a lack of sparse file support, or a PC that is too loud or displays the poor picture because they used a VGA cable insitead of the DVI one...
I fully believe that just like a company or government has different departments that each understand there own work, normal humans only understand there own niche - you cannot be great at everything and without being great you will end up being burned by unreliability and under performance unless you get an expert in the field (a CCTV integrator in this case) to assist.
Yes there are poor integrators out there, but they come and go (hopefully they will find there actual area of strength in another profession) but the majority are proper hybrid security/optical/electronic engineers.
I face a problem in everyday life where I work into a store to buy something and the staff in the shop know just about nothing about the products - and I always think - IF only the vendors chose their resellers more wisely - so much wasted time can be save, and so many less crappy products will end up being produced (because a real integrator will reject crappy products)
I just love selling to knowledgable integrators - they scrutinise our products to the extreme and forces us to weed out underperforming lines, automate procedures that should be automated, and ultimately ensure our long term competitiveness.