I remember being a teenager with a hangover from a night out, and my father making me climb onto the roof the next morning to help him re-roof the house and garage. You can also improve your home's curb appeal with a new roof if you intend to sell it in a few years. Based on the details you've shared, it looks like you have a single-storey ranch with a fairly simple roof (inferred from the square footage of the house plan versus the square footage of the roof). This type of roof has a layer of vegetation that is placed on top of the roof, and is also installed over a waterproofing system.
At the lower end, there may be some roofers who underbid because they are desperate for work or because they work strictly by volume with very tight margins. As with most projects, the more square footage your home has, the more material you will need to replace the roof. I have found that some roofing companies that work in rural areas actually come to town and bid the big dogs out of town. This influences the total cost of the project, as conservatories with unusual slopes or amount of roofing can raise the total cost.
The cost of long-term water damage increases considerably over time, and many roofers do not proceed without repairing existing damage. We knew the roof was old and would need some repairs, but we are being charged almost double the estimate. One important aspect of the estimate that you did not mention is whether this estimate for a re-roofing job comes with a contractor's workmanship warranty. FYI, if your shingles are worn, cracked or curling, this is an important sign that you need to replace your roof.
Roofs are often a good investment in your home, especially if your existing roof is old and your home may go on the market soon. You may be able to convince your homeowner's insurance company to pay to replace your roof, as the roof must pass inspection to be insured, depending on your coverage, they will most likely pay for a new roof. Tariffs have not yet significantly affected material prices, but the rising price of oil, one of the main raw materials used in the production of asphalt shingles, has had an effect on roofing material prices. While this may seem like a lot of money, it is not that much when you consider the work the roofing contractor will be doing.