Roman legionary standards were vulnerable to two main factors that work against material suriviving until the present day:

• They were valuable

• They were reusable

Only a fraction of artifacts made in antiquity survive into the present day. Generally, anything that was valuable or easily reusable was exploited. Metals were molten down for reuse, even less valuable ones like Bronze or Lead - there was an extraordinary amount of bronze tablets, which recorded Roman laws, grants of citizenship, same as lead ones which were used as, f.e., price-tags or for recording debts. Only a fraction of those survive. Stones got re-used, altars, columns, whole buildings were deconstructed, hewn to the appropriate size, and used for building other stuff. Early medieval churches from the former areas of the Roman empire will often be built from Roman building material. Marble statues and altars were broken down and burnt for lime.

Basically anything that could be re-used and was available got re-used. Especially valuable material like gold or silver. The legionary eagles were made from silver in Republican times and Gold in imperial times, so they would have been a prime target for this. Things that survive until the present day often only do so because they were either not valuable or able to be re-used (like pottery), they remained in use (like the Pantheon) or were hidden from contemporary people (burial gifts that weren't plundered, hoards or treasures that weren't recovered, burnt-down and razed buildings, buried stuff in general) or otherwise out of reach (like shipwrecks). It would be quite extraordinary if such a valuable thing as a legionary eagle which could have easily been molten down would have survived. Unless one of them is still buried somewhere (like one of the eagles of the 17th and 18th legion was after the Varian disaster, which Germanicus later recovered).