Is the listed owner Flint Lockwood?
I understood that reference.
I had a similar thought. In a nearby neighborhood, there is a public park where the city-funded pool and bathrooms with it were closed for COVID and have never re-opened. A single porta-potty was added by the city, which is entirely insufficient for the size of the park (multiple groups play team spots there during the summer, there are a few tennis courts, and a large grassed area). One of the parking lots for the park are directly adjacent to a residential area, and right next to those homes, there is an additional porta-potty with a sign stating “this facility is provided by the generous support of a local citizen”.
In this case, they probably got tired of various parents, kids, and normal citizens coming by and begging “just this once” to borrow a personal bathroom, and one or more of them clubbed up to remove or limit the exposure.
May or may not be the same for the OP’s scenario, but a thought.
Maybe it’s bigger on the inside. Or perhaps deeper.
Entry to the Ministry of Magic WC.
That’s why it’s been there so long, and never emptied.
Have you considered walking up to the front door, explaining that you live nearby, and are curious?
Ask them.
Unless they are Ninjas, they wont kill you. 
Or unless they are Texans. You can’t be too sure about many of them.
Doctor Poo!!!
Maybe I live in sheltered suburbia but there’s no way that local zoning laws would allow this. Unless @Asuka is in a loosely governed township or an unincorporated area?
I’m with @StarvingButStrong . It has got to be long term work. Some contractors insist that they will have a portable toilet on site, worked into the contract. Owners may have said “nowhere near the house please!”
I went looking for Irrelevant Astronomy, which used to have some comedy astronomy-related videos, one of which had a character actually flying a porta-potty-shaped space ship. Sadly, the site is now called Cool Cosmos. It is concentrating on infrared astronomy and no longer hosts the old videos.
Pity.
Found the videos on YouTube. The character is called The Physician, which was voiced by Wil Wheaton.
This sounds like they are under some sort of construction permit. Chain link fences in my area are commonlly erected when there is a major remodel etc, and may be required by the local permit. If that’s the case the port-a-potty may be required as well by the permit. The owner may be taking their time with the construction and are willing to pay for the portapotty rental, now it’s permitted, but want to take their time with the design of the remodel, or etc.
Maybe Superman nips in there to change, now the phone booths have all gone.
Sigh…
(Post 15, approx)