"Why did Turbotax make me print form 8453, referencing form 8949 which was e-filed?"
Here's why: On the page where TurboTax asked how you wanted to enter your trades - one at a time or summarized - you answered "summarized", perhaps not understanding the consequences of that decision.
Form 8949 that you submit to the IRS is supposed to show each and every trade you made and each trade is supposed to contain the following information:
IF you opted to summarize your sales THEN you were supposed to make "summary" entries for each of the six sales categories, (Box A, Box B, Box C, etc.), where you had trades. That is for each sales category you'd enter the total proceeds, total cost, total adjustments (if any), etc. needed. These numbers show up on Form 8949 - which is still part of your income tax return - but of course the Form 8949 you submit doesn't contain the required detail. You have to mail the detail information to the IRS submitting either a Form 8949 provided by the broker (although I guess the 1099-B will do as well) or a substitute schedule with the same information.
TurboTax just doesn't do a good job here because the consequences of selecting "summarized" are not even mentioned on the page asking the question. You have to click through two levels of "On Demand Tax Guidance" before Form 8453 get a mention.
I'm also upset with TurboTax offering no warnings about the consequences of using the summary method. All these complaints last year and nothing was done. Fell into the same trap this year.
Just as broken this year as last. And not only didn't TurboTax warn you of the consequences of selecting summary sells, it does a VERY POOR job of telling you what to do with form 8453 & 8949. It prints out form 8453, does NOT print form 8949 and NEVER TELLS you to send in the missing information for each individual sell. It if had not been for this question and answer, I would not have known what to do. I am VERY DISPOINTED with TurboTax and will not use it next year.