It's not impossible, it's just more complicated and expensive. Everything in your house is designed to run from AC. Many smaller products do take DC in but they come with an AC adapter because that's the only available source of continuous, inexpensive power nearly everywhere. The voltage required can be different for each device. The closest thing to a standard for DC power is probably USB 5.0V, which only offers enough current for small gadgets and not anything larger.
The way a solar powered house works is roughly: solar panel to battery charger to battery, to DC-AC inverter to wall outlets, plus another power regulator & meter if feeding extra energy back to the grid, which isn't a requirement. One could power a house directly with unregulated DC from the battery if the appliances were designed to run from it, but most aren't. If the battery voltage had to be regulated before distribution to the house, all you'd really be doing is swapping the DC-AC inverter for a DC-DC regulator, basically a different box with similar cost.
Due to the small size of the market for DC appliances (at the moment), they'd be harder to find and possibly more expensive than AC units. If a time comes when nearly every house has solar on the roof, they might be just as easy to purchase and maintain.
As to reusing wiring, a wire is just a strip of copper and doesn't care whether you put AC or DC on it, IF you stay within its capabilities. If you had to put a lot more current through the wire due to lower voltage, you might need thicker wires, different safety features in the wiring boxes, higher rated fuses and so on. You'd want different plugs on the outlets so nobody made a mistake of plugging an AC device into an outlet providing DC.
Overall, it's cheaper and simpler to put a DC-AC inverter at the battery than it is to gut the entire electrical system of the house and rebuild it, plus buying all new appliances, plus still needing small DC-AC inverter in each room for the devices which can't be repurchased to run from DC - which at the moment is nearly every gadget. You might think of the AC inverter as providing "backward compatibility" with the previous hundred years of electrical devices.
The typical PV installation is either classified as "off grid" or "on grid".
The off grid version charges batteries from the solar panels via a DC bus and an inverter converts the DC bus to AC to supply the facility loads. There is no interconnect between utility company provided mains and the solar system. These systems are often used when utility company energy is not available or it is prohibitively expensive to install and operate.
The on grid version is the most common form. Here a number of solar panels are wired in series and connected to an inverter. The inverter converts the DC bus from the panels to AC. The inverter monitors the utility company AC line and syncs its frequency and phase to the AC line. If the AC mains fail, the inverter has protection circuits so that it drops off of the AC line to avoid causing harm to power line workers, etc. Many installation codes still require a separate mains disconnect switch, however. This type of system supplies all of its energy back to the power utility and the facility loads remain connected to the utility. This essentially allows the solar produced energy to offset the purchased energy.
An on grid system can include any number of inverters to reach the desired energy capacity. No special connections are required as they are typically paralleled on the utilities side and connected to a unique string of solar panels on the DC side.
Either style of system can employ MPPT (Maximum Power Point Tracking). This is a technique that extracts the maximum power available from the solar panel under the current sunlight conditions. Some newer systems put small MPPT trackers in place for every solar panel or a small collection of panels with the promise of further optimizing the total solar panel array power when some panels are shaded by a tree or cloud for example.
A few on grid system offer an accessory connection to the inverter for a few kW of power to provide a limited form of backup for the facility when the mains fail. This is rarely sufficient to power all facility loads.
Best Answer
Its not free energy to the system.
What the ideal current source does is facilitates forcing current around the loads & from the supply.
If there are loads forcing current to flow, the resultant VA can be measured and the affect on voltage distortion can be seen at the utility