assessments, same as any person. But there have always been both atheists and people who believe in superstitions, supernatural forces and beings, and sacred legends and sites.
As atheists point out, atheism is disbelief, and not a system of beliefs. So what motivates and unifies people with a sense of purpose and meaning to return to the same site, year after year, and carry out a building project that requires a lot of physical labor and manpower, but is not a residence?
Ceremonial and religious customs are one possibility and there is evidence that could support that so archaeologists are not just pulling that hypothesis out of a hat. Gathering together as a group at one site year after year can be a ceremonial act. Religions are full of sacred pilgrimages, from Hinduism to Christianity to Islam, plus various tribal religions
Many Native American religious ceremonies and rituals prior to Europeans (and today among some) are experiential, not ideological, and serve(d) as unifiers that also express commonly shared cultural, world, and cosmogical views. That is typical of tribal religions. Traces of that kind of experiential religion continue in Judaism today, which was founded among Hebrew tribes
Ceremonies and rituals do not require an altar. Shaping and carving stones that are not foundations for residential buildings, but only for yearly gatherings, can be ceremonial. Pilgrimages to special (sacred) sites are ceremonial.
I am not saying that's the case with these stones and the repetitious gatherings. I am just answering your point about no evidence of ceremonies.
Another motivator to return to the site periodically would be for food. It is possible that certain plants were ready to be gathered at that location at a certain time each year. Or herds that provided the people with meat, hides, bone tools, etc. might have gone periodically to that region. The OP mentions an abundance of bones. Soil sample analyses of any pollens plus examination of bones could support that. But, where were the families? Did the men take food back to them? Was it exclusively a male site?
My next point comes from being female. The OP describes and shows phallic symbols. In other societies that have had phallic symbol images, they have been associated with religious belief. No woman can find that hard to believe about men considering their genitals sacred.
Regarding your allusion to human sacrifice, that is usually associated with either agricultural societies as part of a fertility ceremony, or in more densely populated sedentary cultures.