Seeing as Umbreon is one of the most popular (and valuable) subjects of Pokemon TCG cards including in the most valuable Eeveelution cards of all time its no surprise that this card from 2010 commands a near-$30 price point
His tap-to-destroy mechanic adds control elements while scaling with each creature fed to him
The difference between these two methods is explained in detail on The Collation Project
Giving them a card or a creature token is much more of a cost
Baymaxs ability, The Treatment Is Working, allows him multiple other avenues to gain extra lore for his controlling player when Baymax heals his allies, which is done by just playing Baymax initially and whenever Baymax quests
many collectors have little-to-no interest in using their cards for gameplay, instead focusing on acquiring cards based on rarity/market value, a desire to complete a particular set, or a fondness for a certain Pokmon species or a specific art style