It's more because Burst damage is a "either if" choice there. I mean, if we compare a SR Assault 13% with SR Assault+Exceed which is 10% Assault, 30% Burst damage and 200k more cap, it's not a bad deal.
Meanwhile, in (non-Ancient rolls) Accessories, it's 3% Assault vs. 5% Burst damage. Why wouldn't you always rather want Assault which works every turn, instead of only on your Burst turns? Not to mention that it's not a "burst damage * 1.05", it's "burst damage modifier + 0.05" which for SSRs who already have 500%, won't make much of a difference. In fact, the 3% Assault is very likely to be more damage on your Bursts than 5% Burst modifier. I can't be bothered to run the maths, but let's say that should always hold true at least with PF on SSRs.