What a few people above seem to forget is that the two manufacturers, like most businesses, are run by accountants.
Low-cost and low-risk is inevitable in such a situation.
In the last 30 years, Boeing has commercially launched one all-new aircraft, the 787 in 2003. Airbus has launched two, the A380 in 2000 and A350 in 2005/2006*.
*The A350 initial launch was for a beefed up A330 with a new wing and engines which received a pretty poor reception from potential customer so Airbus went on to reformulate their concept and relaunch the following year.
Everything else has been a development of an existing product.
A 757-200 can also fly farther than an A321neo. And the 757 uses the same airframe as the 737, so restarting and moving production to Renton wouldn't have been difficult.
This isn't true (with a huge proviso).
The Boeing quoted range for the 757-200 is 3,915nm. The Airbus quoted range for the A321XLR is 4,700nm. A significant increase.
The huge proviso being that manufacturer quoted range is not close to the real operation range. The manufacturer's range is based on a very lightly loaded aircraft (a light seating configuration and little to no cargo) in ideal wind conditions. It's like car manufacturer's range/ fuel consumption numbers.
The longest A321 flight currently operating is longer than the longest 757 flight.
It's already been answered above, but the 757 doesn't share an airframe with the 737. They have the same dimensions. Every Boeing narrow-body jet* has been based on the dimensions of the 707, but were not closely related otherwise. The 757 was developed alongside the widebody 767 and the two shared many components and systems.
*The 717 was a McDonnell Douglas jet rebadged after they merged.
A pilot with an A320 type rating can fly everything from an A318 through to an A321XLR and an A330 rating can also fly A350s. So most airlines with a purely Airbus fleet only need their pilots to hold two ratings.
Meanwhile at Boeing the 737, 757 and 777 all need different ratings. And one of the issues with the MAX was the bodging of its flight physics to keep it covered by the existing 737 rating.
This is a gross simplification and, in many respects, incorrect.
Of operational in-production commercial aircraft, Boeing has two (+2) type ratings and Airbus has two (+2) type ratings.
The Airbus type ratings are the A320 family rating and the A330/A340/A350 rating. The +2 are the A220 family which was developed by Bombardier and the programme then bought by Airbus as Bombardier collapsed and the A380 family which is soon to end production.
The Boeing type ratings are the 737 family rating and the 777/787 rating. The +2 are the 757/767 (the 767 is still in production) rating and the soon to end production 747-8 rating.
Someone with an A330 rating may not necessarily be allowed to fly an A340 or A350 despite them being the same rating as they would be required to undertake differences/ familiarisation training and, likely, some addition sim work as required by their authority.
Someone with an 737 rating might not (will not in reality) be able to fly all 737 versions. Generally, the rating will cover one or several of the families within the family (the MAX, NG, Classic etc. families). When the MAX was introduced, pilots had to do a a very short differences course before flying it. Boeing did everything it could to limit the length and difficulty of that course to limit implementation cost and difficulty for airlines. Now we know that didn't work out too well, but Boeing's plan would be no different to Airbus' or any other manufacturer.
The 757 was also an expensive dog to manufacture. Analysts at the time reckoned the manpower cost to build one was greater than that for the widebody 767.
Do you have a source for this?
The 757 and 767 were built on the same production line and shared many of the same components. I can not imagine it would have cost more than the 767 to manufacture.