Because, you see, either pronoun is absolutely correct in that usage.
Ed Greenwood (whom I’ve known, literally, for decades, and whose work I’ve edited) asked me to write a post about “all the folks who use ‘that’ for people, instead of ‘who.'” I have to presume, therefore, that Ed (I LOVE YOU, ED) eschews the usage of “that” for people.
I hate to burst your BubbleYum, old friend, but it doesn’t matter.
Here’s what Bryan A. Garner says, in Garner’s Modern English Usage (4th ed., 2016): “Is it permissible to say people that, or must one say people who? The answer is that people that has always been good English, and it’s a silly fetish to insist that who is the only relative pronoun that can refer to humans.”
Then there’s this, from The Merriam-Webster Dictionary of English Usage (1999): “Summary: In current usage, that refers to persons or things, which chiefly to things and rarely to subhuman entities, who chiefly to persons and sometimes to animals. That is definitely standard when used of persons.”
In his Oxford English Grammar (1996), Sidney Greenbaum tells us “That and the zero relative pronoun … are used for both personal and non-personal reference.”
I think this next entry is quite interesting, mainly because I’ve not seen it anywhere else except in this discussion from Pullum and Huddleston in A Student’s Introduction to English Grammar (Cambridge, 2005). It’s talking about integrated relatives: “[That] is used as a neutral way of sidestepping the choice between the distinctly formal [whom] and the distinctly informal [who}.” The exemplars are: the applicants who we interviewed (informal), the applicants whom we interviewed (formal), and the applicants that we interviewed (neutral). Reading the exemplars near the one I just quoted confirms that Cambridge embraces either one as acceptable. “Anyone who wants this stuff can have it.” “Anyone that wants this stuff can have it.” “some friends that saw her”
Well, there you have it, Ed. I have a suspicion this wasn’t quite what you hoped for, but I have to report the facts, y’know.
Besides, I got to quote Garner and use the word “fetish.” Surely that makes up for some of the disappointment, right?
Grammargeddon! 
No, you covered the distinction I wanted “put out there.” (The Pullum and Huddleston example.) :}
It will save me from trying to parse sentences like this one, from a recent newspaper (Canadian, but captions done by a service in Pakistan): “Shown are the crowd that heckled the visiting statesman and the same crowd that supported him, excepting those individuals that were unaware of his identity and expressed some bewilderment.”
I.e. fetishistic use of “that” to avoid who/whom decisions, when the sentence would flow much better by using a “who” and some rewording.
Almost as bad as some of the Chicago papers I read on my last passage through O’Hare, that used “who” for things. (“The trash bins who always seem to end up in the streets.” Bad, BAD trash bins!)
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Unless, of course, Chicago has SENTIENT trash bins and you’ve all just been keeping it secret from the rest of us, lest we avoid your fair city out of naked fear…
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I discovered this site via Lisa and her Blog tour. This post answers a question I had about using who and whom in the context you mention. It’s nice to know that informally I can use ‘who’ in that way and yet that ‘whom’ is correct as I expected. But the big plus is the use of ‘that’ in such a case. No one ever calls me on any of it, but this will help me not judge others so much. And that is a very good thing.
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