"I know I have to clear customs in Frankfurt. . . "

No - you won't clear 'Customs' in Frankfurt.

You clear 'Immigration' (Passport control at Frankfurt and Customs at Venice. You won't see your luggage until Venice (it will be checked through)

I guess I must be mistaken since you normally clear passport control and customs at your first arrival point in the EU, at least you do when passing through Spain, France and the UK.

You clear Immigration when entering Schengen (in this case Frankfurt) and clear customs at your final destination (Venice) . . at least that's the way it has happened every time I've transited in Europe.

No, you won't have to clear customs.

We check our bags and they always tell us, "your bags are checked through to..."

Now on the return home, it's another story. A very stupid story imo.

We've done many trips on United/Lufthansa from the US through Frankfurt to other destinations in Europe, and our luggage has always been checked through.

janisj is correct. Germany and Italy are both part of the Schengen area, so you go through immigration/passport control (and also security) at your point of entry in Frankfurt. Forty-five minutes is doable (if your first flight is on time) but tight. Be prepared for the fact that one or both of your flights will use a remote stand and bus to/from the terminal.

I guess I must be mistaken since you normally clear passport control and customs at your first arrival point in the EU, at least you do when passing through Spain, France and the UK.

The EU and Schengen are different things. If you were flying to Spain or France via the UK then you would pass through passport control in Spain or France as the UK is not part of Schengen.

I have never cleared "customs" on a connecting flight through Europe with a destination in Spain.. only passport control.

Even when connecting through Heathrow to Amsterdam I've never had to collect bags and go through customs. Passport control (maybe? I can't remember tbh) and security yes (that I do remember!!), but not customs.
Within Schengen you go through passport control at your point of entry into the Schengen zone, and through customs at your final destination.
It is something which America feels the need to do but Europe is happy to wait until you get to your destination. And even then it is normally a non-event. You don't even notice you are going though customs unless you are behaving suspiciously when you will be picked on.

We can only wonder why? Oh why? Does America feel they need to do that???

Because many of the airports that people will be flying into are NOT international airports - but domestic - and don't have facilities for either immigration or customs.

I know this is different than europe - but there are a lot of airports here (including the huge LGA in NY) that are for domestic flights only.

>>>>>>We can only wonder why? Oh why? Does America feel they need to do that???

Because if they did, they would have to have Customs officers in every podunk airport from Missoula to Billings to Pierre, North Dakota to Grand F**ks, South Dakota. It would cost a bleeping fortune.

It'd solve the unemployment problem though .

It's the reason when we fly to the US we try and do any transfers within Europe rather than in the US.
Made the mistake once of doing the transfer in the US. Was not fun - an hour in the immigration queue, then claim baggage, go through customs answering more questions with a jet lagged brain, rechecking bags and finding the gate for the next leg of the flight.Never again!

Thanks. So, is 45 minutes enough to get through passport control in Frankfurt. If not, then why does United even offer this as an itinerary?

45 min? May be. I would be comfortable with 45 min at AMS, but not at FRA. I think your FRA-VCE flight is on Lufthansa? While the flights from the US often arrive earlier than the schedule in Europe, it probably does not materially change your short layover. Ask the flight attendant what to do, and then there is usually airline reps as you get off the plane to deal with people with short layovers like you. FRA is a huge airport and you probably need someone to help you expedite your transfer process.

Why do airlines offer certain itineraries? It is probably safe to say that doing so improves their profit even considering the cost of dealing with missed flights. They really don't care about your inconvenience. It is up to the buyer to decide which itinerary is acceptable. When I see an offer like this at problem airports, I play with the booking engine to see if it can cough out a better itinerary with a less risky layover without significantly increasing the price.