Comment by jccooper

Generators are typically grid-forming. They can (and will) be out of phase with the grid power, and will backfeed and create hazardous conditions upstream if the grid is out (and your main breaker is still on.) To use a generator safely you need a transfer switch (or similar) to make sure only one source of power can be active at a time.

Solar inverters (esp for this use, but also in general) are grid following; they'll use the existing frequency and support it, and if the grid goes off they'll go off too and do no power-off backfeeding.

The inverters are thus broadly safer to be plugged in to systems that are otherwise unprepared.

Generators could be made grid-following (many are now inverter-based), but for most users that would be silly, since the supplemental power would be more expensive (and louder!) than from the grid.