To add to what aszantu said: plants with deep roots can be very healthy for a garden because they essentially draw nutrients that have soaked deeper into the soil back up. So one way of looking at them is that they are basically mining soil nutrients from below for your garden for free. Perhaps that will make repeatedly mulching them a less frustrating task.

Also, while there are of course legitimate reasons to consider certain plants weeds (e.g. they maybe be poisonous, toxic, or displace other plants that you like more), dandelions are mainly the victim of marketing from pesticide manufacturers half a century ago.

they're amazing, you can make salad with lemon and salt, or fry in butter and eat with your steak. Also the roots are edible. And dandelion pulls nutrients for other plants from below, once the nutrients have been used up, the dandy will leave on its own.

Those are extreme examples, most probably in sand or a loose mix. Roots need oxygen also so can grow a lot in this conditions as long as they are watered.

Most dandelions live in heavy clay. There will be much shorter, reaching just the phreatic level.

This was an extreme example that I found in this collection. Here's something more common (20 cm): https://images.wur.nl/digital/collection/coll13/id/576/rec/1

But there was also a specimen with over 2 meters: https://images.wur.nl/digital/collection/coll13/id/550/rec/3

It varies, but yeah, they are surprisingly deep.