Neither are huge stars and the idea that Tom Holland is some a-list celebrity that she would need to be with to get famous is absurd. Very few of the MCU are actual box office draws when they're not playing their respective characters. Look at Chris Hemsworth and how well the MiB or Ghostbusters reboots did.

Zendaya is at least big enough that when she was cast as the lead of an HBO series she was treated as the main draw to the point where the promo art was just her face as opposed to marketing it as an ensemble piece that she was part of.