Why would you not plant a garden? I could maybe see not wanting to plant root vegetables, but peas, strawberries, cabbage, etc surely present no issues whatsoever. (And TBH, root vegetables should be just as fine if the septic tank is doing it’s job.)

From what I read about it, for three reasons:

    It's not entirely clear (some people/studies say it's fine, others say it's not) whether certain pathogens can end up in your vegetables this way and/or which vegetables would be fine and which wouldn't. I mean, some people do compost their own #2 and put it on their veggies and while it's been OK so far for many, I personally stay on the safer side of these things until desperate times call for such measures.

    Anything that has a root system that is able to reach the septic field's drain lines is potentially able to clog the holes. E.g. I tried raspberries in a planter on top of it for a while but even though I tried, I couldn't contain the roots and/or raspberries fall down and the seedlings grow in the grass. I ended up removing the whole thing just in case as septic drain field repairs are either expensive or a lot of work.

    Evaporation apparently plays some part in the effectiveness of the drain field (it doesn't just all drain out into the ground). Though I wonder, why plants taking up the water and evaporating it via their leaves wouldn't do the same thing, potentially even better through a larger surface area, so I guess it's mainly the first two points.

Well, anecdotally, I ate from a septic-fed garden for more than a decade, to no ill effect, from potatoes and carrots, to peas and broccoli. I wouldn’t hesitate to do it again. (But, yes, one does need to consider roots clogging the pipes; annuals perhaps safer than perennials, which surely drive roots deeper.j

Clover is a popular alternative. Some homes are xeriscaped (no lawn). But converting an existing lawn is expensive, even with subsidies. Selecting native grasses and watering less is an affordable compromise.