Two of my children married their platonic friends, when they realized that they had developed feelings of attraction to the friends they really cared about.
Obviously then it wasn’t so “platonic” in the first place! And if it were, it sure serves as additional proof it doesn’t remain too “platonic” for long. Who knows how many sins occured — without your knowledge — prior to this so-called “platonic relationship” resulting in marriage. Sure, in this case it ended in marriage, but in so many more cases it sure doesn’t.
And that is why “platonic relationships” are assur m’doraisa. (See above posts for sources.)