as mentioned stuff is welded down both on purpose and quite often accidentally
but the accidents generally lack any real penetration, sometimes its more important to get that nice clean weld in a corner and the slight overspill tack to the table can either be cut\ground or cracked off
the main reason you want a good thick steel tabletop is rigidity, welds "pull"
a slight deflection isnt that big a deal on a small scale, but a long railing, it starts to add up.
"pro" tables are often a thick gridwork
Acorn Welding Table
where pretty things get welded, no tacking, adjustable dogs\clamps\ect
but we pound, grind, and dent all our pretty metal before it even starts to get welded, so a solid top is much cheaper