"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.