Why didn’t G-D teach the ancient Israelites to have more respect for women. HE never reproached men for have multiple wives and concubines. He never reproached people for giving their children away to be married to strangers.
It seems pretty likely that in the ancient Middle East, a woman was generally better off having a man provide for her. Yes, the past is a foreign country.
If you look at the way Exodus 21 actually describes "selling" a daughter (it immediately becomes clear this was more like a ruse for arranged marriage), the thrust of the verses is -- if you're going to do this, there have to be a lot of protections in place -- don't do it this way, don't do it that way ... (and then the Talmud reads even more caveats and provisos into it). The implication is that the Torah is moving people away from the common practice, which had little to no protections for the women, towards one which had more.
Many Jewish thinkers have in fact suggested that given the mores of the time, banning these practices outright wouldn't have worked; instead, enough limitations were put on them so that people would eventually be weaned off them entirely.
The questioner is bringing a modern mindset and plugging it into biblical times. While it may not be acceptable these days, back then, the whole world over, a single woman had no respectable status and no source of income to be self sustaining. If a man married a woman, she was taken take of in both of those categories.
So back then, not only wasn't it disrespectful, it promoted her status to being married which was looked at as respectable.
It might be true to say that it wasn't ideal to be married to a man with other wives, the gemorah calls the term a "tzara" which is something like a competitor or a nuisance to the other wives, but it was probably better then the alternative of being single.
So if you don't find it respectful, it's probably because these days you have more options so if it were legal it would not be respectful but it's not legal or acceptable in the jewish world these days. (There may be a handful in some third world countries that have more than one wife but it's not mainstream in most of the jewish world.)