If you wanted to develop a marketing agency 25 years back, the only obstacle to entry was mammoth.
On top of all the startup expenses, you were pretty much limited to conventional and physical media, and even then you were operating against a brick and mortar infrastructure that may as well have been the equivalent of having a local brick and mortar business.
The internet, of course, has made the world a much more competitive place in which to do business, and it is now a great deal less difficult to Gawdo than it has ever been before.There are still plenty of agencies out there that have not yet caught up to the times.
Those that have are probably more focused on staying relevant and current, and have not developed truly comprehensive marketing strategies.
These companies will find niche markets for their clients and work relentlessly to help them build those niches.
Think about it - if you don't sell something online, chances are good that nobody else will either.