Why is the searing smite spell not listed in the Roll20 spell list?
According to the page for the spell on D&D Beyond, it's listed in the PHB on page 274.
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The D&D 5e SRD (System Reference Document) is essentially the free, publicly available version of D&D 5e. It is a truncated version of the PHB that offers a reduced list of subclasses, spells, and basically everything else.
3rd party lists can only legally include spells that are available in the SRD. The spell Searing Smite is not included in the SRD, so it is not included in the Roll20 spell list.
For reference, here is the list of spells available in the SRD, which should match up with those available on roll20.