Can large ISPs really offer 1Gb “dedicated” service to every home and
Yes, of course they can, much the same way a business can give every PC user in the company a dedicated 1 Gbps connection to its network, but not the total of all the user bandwidth to the another LAN or the WAN. Remember that the Internet is really just the ISPs peering with one or more other ISPs. The connections between an ISP and any other ISPs with which it peers are almost certainly not up to the total bandwidth it offers to its customers.
This is called oversubscription, and it has been around ever since there was telephone service. The telcos, and now the ISPs, count on not every customer using the full network capacity at the same time. Based on studies, the telcos came up with formulae that determine the proper amount of oversubscription. You would rarely get an "All circuits are busy" message, but it still happened. Good ISPs do something similar, and crappy ISPs will be too oversubscribed. This is where the Net Neutrality debate comes in.
The ISPs are selling you bandwidth to their networks, but, unless you are a business with specific contractual agreements, they make no guarantees about the available bandwidth beyond that.