They are typically first wired in series to increase the voltage as early as possible, to make them safer. If you were to wire them in parallel you would keep a low voltage and increase the amperage, which would make their wires glow like Christmas lights.
That's the whole reason why they started selling micro-inverters that go on each panel: to increase the voltage and decrease amps before even leaving the panel area, to make them safer.
Look at a typical micro-inverter: input is up to 50 volts, 10 amps. Output is 240 volts, 1 amp. https://solartown.com/solar-products/enphase-iq8-microinvert...
For panels, the voltage is not dangerous part, the intensity is.