Because this is the default vertical alignment for tables. You can see the Specs for Visual Formatting Model at W3.org or Vertical Align at MDN
The following values only have meaning with respect to a parent inline
element, or to the strut of a parent block container element.
In the following definitions, for inline non-replaced elements, the
box used for alignment is the box whose height is the 'line-height'
(containing the box's glyphs and the half-leading on each side, see
above). For all other elements, the box used for alignment is the
Align the baseline of the box with the baseline of the parent box. If the box does not have a baseline, align the bottom margin edge with
the parent's baseline.
As for WHY was this default chosen, it's because it's easier (or at least it was) to read adjacent content by considering the end of each cell. This comes from old accounting books, see example below, you'll see the amounts are at the baseline of the content of left column (which is multiline).
However, keep in mind that this behavior is agent dependant, meaning that browsers, programming languages and software can declare whichever default they want. For example, Latex aligns to middle and old MS DOS software usually aligned to top