2008 to 2013 is only five years. An extra five years would not magically make WB start putting out movies at the same rate that the MCU does.
There are only 15 films in the entire DCEU (ignoring alternate cuts, anyway), released over ten years. Marvel is up to 37 (and a ton of in-universe shows, too) in just 17 years.
That's not even a knock against WB. What Marvel Studios does is genuinely unique and arguably kind of insane. The DCEU was already very fast-moving compared to most other film franchises, let alone the MCU.
Bottom line, certainly a decade ago at least, DC were absolutely not in a position to even attempt something like that because that sort of thing simply is not possible under the traditional studio model of WB. We'll see soon enough whether Gunn's new status is actually independent enough to surpass those limitations for his new DCU, but even if it is that's just another thing that simply could not have happened under WB in 2008.