Any seats where the belts are attached is going to be heavy, simply because it has to withstand enormous loads in an impact.
Having removed many seats over the decades, I've experience a massive shift in weights of seats, from the 70s where rear seats were little more than a light weight mattress shaped to fit a car, all the way modern 8 motor front electric seats with integrated belt anchorages and 2 air bags, which are a 2 man lift as they're so heavy.
It's no wonder that modern cars are so heavy, as crash worthiness dictates items in the vehicle which keep passengers safe are super strong.
Slightly off topic, on another LR forum recently, there's been a thread about EVs, and how the tyres are more pollution than diesel (completely wrong) but the cause of this excess tyre wear is due to the weight of an EV. Now this forum is an LR forum, but it seems that most people on there don't have a clue about the weight of a modern LR, and then are pointing fingers at EVs because they've heard that EVs are heavy. I wonder how many would admit being wrong about this, if they were aware of just how heavy their beloved LRs are.