If you lose control of an equipped creature, the Equipment stays attached to them, but you may still move it to one of the other creatures under your control at sorcery speed
To have a Tribute Summon theme, they needed to have access to the monsters themselves (gated by an initial Tribute Summon due to the searcher Return of the Monarchs ), they still needed to have the initial tribute fodder to begin with, and with the rise in importance of monster effects, they needed ways to combat the ever-growing arsenal of responses to monster effects
While most colors have access to removal in some form, White is the master of Exile
Blue Serum Visions doesn't look like much at first glance, but the fact that it replaces itself while allowing you to manipulate the top of your library makes it a powerful card that is good in all stages of the game
The reason Shorikai needs the can be your commander text is that its not natively a creature, something folks might gloss over because it has a power and toughness prominently displayed