I don't see where Quicken is hiding anything.
"Why don't realized capital gains from mutual funds appear on the investment activity report ... ?"
If you are referring to mutual fund "distributions" of their capital gains:
- those are not your "realized capital gains"; the gains belong to the mutual fund company, who is "distributing" those gains to you - so you get to pay the capital gains taxes on those gains
- capital gains distributions appear in the "Dividends" row in the Investing Activity report (Q2017 and Q2019)
"Why don't realized capital gains from mutual funds appear on the ... capital gains estimator?"
a.) how would Quicken estimate the capital gains a mutual fund was going to incur and how much of that gain would be distributed to you? Quicken does not know what securities the mutual fund owns, when those securities were purchased, or how much was paid for them.
b.) even if possible, what value would such an estimate have?: unlike the securities you own personally, you have zero control over the mutual fund company's choice of whether to sell, what to sell, or when to sell
c.) once the capital gains have been realized by the mutual fund and your share of the gains has been distributed to you, there is nothing left to "estimate" - your share of the amount of the capital gains is known, and reported to you as short (and/or long) term capital gains distributions.
[When initiating a discussion, you should include your Quicken year, edition, and release; as in: Q2019 Deluxe R16.14.]