Why can you not share a gold sticker if you have extra?

Gold stickers can only be traded or shared during a Golden Blitz event. These events happen infrequently (sometimes every few weeks). Additionally, not all golden cards can be trading during a Golden Blitz event. To date, only two stickers are chosen by the game-makers during each Golden Blitz. During the course of a season (the predetermined period allowed to complete your album) you may not see all Golden stickers being traded.

Gold stickers are only available to trade or gift during a Gold Blitz and then only the gold stickers that they include in that blitz (I’ve only seen 2 gold stickers per Gold Blitz that become available for gifting or trades) you can check out the Gold Blitz schedule tha App Gamer has under events. He also usually posts predictions for future Gold Blitzes on the forum