The simplest answer to why you can't see it in your online statement is a design/business decision that was made, most probably originally to make online statements differ as little as possible from old fashioned monthly printed statements; the old printed statements never showed holds either.
Some banks and card services actually do show these transactions online, but in my experience these are the rare exceptions - though with business/commercial accounts I saw this more, but it was still rare. This is also partly due to banks fearing lots of annoying phone calls from customers and problems with merchants, as people react to "hey, renting that car didn't cost $500!" and don't realize that the hold is often higher than the transaction amount and will be justified in a few days (or weeks...), etc - so please don't dispute the charges just yet.
Behind the scenes, I've had bankers explain it to me thusly (the practice has bitten me before and it bothered me a lot, so I've talked to quite a few bankers about this):
There are two kinds of holds: "soft holds" and "hard holds". In a soft hold, a merchant basically asks the bank, "Hey, is there at least $75 in this account?" The bank responds, and then has it's own individually set policy per account type as to how to treat that hold. Sometimes they reserve no money whatsoever - you are free to spend that money right out and rack up NSF fees to your heart's content. Yet some policies are to treat this identically to a hard hold and keep the money locked down until released.
The hard hold is treated very much like an actual expenditure transaction, in that the money is locked and shown as no longer available to you. This varies by bank - some banks use an "Account Balance" and an "Available Balance", and some have done away with these dual terms and leave it up to you to determine what your balance is and what's "available" (or you have to call them).
The key difference in the hard hold and a real expenditure is, technically, the money is still in your bank account; your bank has merely "reserved" it, earmarking it for a specific purchase (and gently promising the merchant they can have their money later), but the biggest difference is there is a time-limit. If a merchant does not process a completion to the transaction to claim the money, your bank will lift the hold after a period of time (I've seen 7-30 days as typical in the US, again varying by institution) returning your money to your balance that is available for purchasing and withdrawal.
In every case, any vaguely decent banking institution allows you to call them, speak to some bank employee, and they can look up your account and inform you about the different sort of holds that are on your account that are not pending/completed purchase transactions.
From a strictly cynical (perhaps rightly jaded) point of view, yes this is also used as a method to extort absurdly high fees especially from customers who keep a low balance in their account. I have had more than one bank charge NSF fees based on available balances that were due to holds made by gas pumps, for instance, even though my actual "money in my account" never went below $0 (the holds were for amounts larger than the actual transaction). And yes, the banks usually would waive those fees if you bothered to get someone on the phone or in person and made yourself a nuisance to the right person for long enough, but they made you work for it. But I digress....
The reality is that there are lots of back and forth and middle-men in transactions like this, and most banks try to hide as much of this from you the client as possible, partly because its a huge confusing hassle and its part of why you are paying a bank to handle this nonsense for you to start with.
And, as with all institutions, rules and policies become easily adjusted to maximize revenues, and if you don't keep sizable liquid minimum balances (100% of the time, all year long) they target you for fees.
To avoid this without having fat wads of extra cash in those accounts, is use an entirely disconnected credit card for reservations ONLY - especially when you are traveling and will be making rentals and booking hotels. Just tell them you wish to pay with a different card when you are done, and most merchants can do this without hassle. Since it's a credit card with monthly billing you can often end up with no balance, no waiting around for a month for payments to clear, and no bank fees!
It isn't 100%, but now I never - if I can possibly avoid it - use my debit/bank card to "reserve" or "rent" anything, ever.
