Why doesn't sunfire applies spell effects like thornmail?

Honestly I would buy it on just about everyone

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Every game on tanks like You really want 4% hp burns with slows and sunfire on tanks every game?

They have 3 items， if you take boots into count thats 4 item slots with just 50armor and zero MR， unless they are already super ahead， they will fail their job as a Tank and there is nothing to worry about