Framing question: If you already accept that it is not paid, what makes the weekend less "ethical" compared to off-time during the week? It maybe more practical for you to join during the week, but perhaps it's more practical for your colleagues at the weekend.
Answer: This really depends on the type of team event. A bowling evening can perfectly be done during the week. A two-day rafting excursion not so much. Even if the company would be willing to use working days for it, once an overnight-stay is involved it is typically easier for employees to manage attendance on the weekend (e.g. no need to drive kids to school).
The trade off in most companies I've worked in for team events is that it's paid by the company but done off the clock. There is the rare exception where it's partly or fully company time, but not necessarily during normal working days but rather on a weekend.
There are benefits doing an event during the week and doing it on the weekend. During the week, small events after work are typically attracting more people, as it's easy to just go there after work. For late night events, company parties or multi-day events, the weekend is typically more practical (if announced well enough in advance) - especially if it is not work time, but also otherwise (depending on location and work force perhaps).
You didn't tell us how the event is organized. In many companies team events are managed individually by the teams and thus it is often formed some team consensus when to do it, often organised by the team manager or even a regular team member. At other companies - or generally for company wide events, there is more central planning which is less flexible.
So what you describe can be completely normal. Depending on the type of event, it can also be a bit unusual to do it on the weekend. If it is organised on a team level, maybe find out who is organising it and provide feedback that you would be more available during the week. You might learn why it's scheduled on the weekend (perhaps all others in the team prefer it that way?) and you might open up a discussion to change the date. If it's centrally organised for the whole company, you can still provide feedback, but obviously this is less likely to change anything, as this is more likely a general company policy.