This issue that you have brought is not really answerable here on a forum, but if we wish to discuss reasonable possibilities well..............
First, lets' say that often a failure will have more than one parent, so be prepared to realize that perhaps no one subject addressed may solve the issue.
Next, don't think that all bronze alloys are soft, some of the aluminum-silicon bronze guides that are available will outlast the stainless steel chromium valve stems! What those particular Edelbrock heads might have in place?
Are the valves defective? Definitely suspect but this would prove complicated to establish, rather this would generally be determined via historical service record of in service performance in comparison with other production runs, and this information would generally be best acquired from Edelbrock via feedback from customers, if they were willing to share such, this being questionable particularly if it were to present their product in a bad light.
Insufficient lubrication? Sure seems a possibility looking at the damaged, mauled valve stem surface. This could be caused by the guides being to tight thereby not permitting adequate oil passage (it does sorta appear that maybe the top of the guide might be without a chamfer on the I.D.?, which I would prefer to be present to aid in oil migration); oil seal function failure, say for example if not defective in some way perhaps a mixture of 11/32" seals with 3/8" valve stems stuck thru them; excessive oil contamination from overly rich fuel mixture defeating the lubricity value of the oil; how-a-bout bad valve train geometry leading to among other things excessive side-loading of the valve stem in the guide causing displacement of the film thickness required to keep the two surfaces apart; or even maybe overly restricted oil delivery to the upper valvetrain area thereby there's just not enough lubrication to dole out?
But no, I can't answer the question with any absolute, particularly from here, and the culprit may very well prove to more than one issue at hand?
Scott.