One partial explanation is some shareholders may be elderly, and...

One partial explanation is some shareholders may be elderly, and struggle to read the documentation, especially if they have developed dementia.

A more common cause may be that some perceive they don't have the funds to purchase shares via the SPP. But they could sell some or all of their existing holding to finance the purchase, and end up 'ahead'. (There would be CGT ramifications, but that's hardly bad if one makes a profit, and if not, any loss can be written off against profitable other share sales in a FY).

Another contributing factor may be how some investors (hard as it is to imagine) do not read emails or mailed letters.

There will also be a few who were overseas and who didn't keep on top of emails (and lacked access to what Australia Post was delivering to their residence or PO Box).

But this isn't exhaustive, and like you I am bewildered by those who knock back 'free money'. Most of us aren't investing just because we're altruists.