Well, the actual reason is probably because there wouldn't be a ton of plot if that was the solution the creators went for. It also wouldn't create the plot they were going for, which was Lightning trying to keep up with The Youth™, realizing and accepting he couldn't, and passing the baton to the next generation.
Because it wouldn't change the fact that McQueen, as an athlete, is getting older. As far as I know, the notion of McQueen getting mechanical upgrades to keep pace with the new racers isn't directly addressed in the film, so the flippant answer (as already commented) is "because plot".
Why didn't McQueen cross the finish line before helping the king? Because helping him was more important than winning. Helping someone who is older than you is more of a reward. If you want to get technical, the racing world would have viewed Lighting as a jerk for not helping a legend like King.
Meanwhile, Lightning is cheered and congratulated for his act of good sportsmanship. Tex, head of the Dinoco company, offers Lightning a sponsorship because of his fine job. Lightning considers, but decides to stay with Rust-Eze because they gave him his big chance.
In the final race of the season, McQueen was about to make history by becoming the first rookie to win a Piston Cup, but during the final lap of the race he blew his rear tires and on the final turn he TIED with The King and Chick Hicks.
Why couldn't McQueen just get an upgrade to become faster?
Could McQueen have beaten Storm?
Speed wise, McQueen could never hope to be a fast as Storm. However, that doesn't mean he couldn't have beaten him at least once or twice. In one race of the montage - McQueen comes a close second to Storm which heavily implies that Lightning has the ability to temporarily overtake Storm at speed via drafting.
All Chick Hicks needed was Dinoco which he did not get. He is just a cheating jerk and selfish car, not a villain. He did not get Dinoco, because he cheated to get the Piston Cup by purposefully doing a PIT maneuver on the King.
Not Chuck, wanted to be rather referred as Charles, is a small forklift who worked on Lightning McQueen's pit team, before he and the other pitties quit as a result of McQueen's disagreeable attitude.
Why Is Lightning McQueen #95? The racecar's number was originally going to be 57, in reference to the birth year of John Lasseter, who directed the first two Cars films. In the end the number became 95, which alludes to the year Toy Story debuted.
This became a problem, because cars don't have hands. They can't open their doors, because that will show their brain. His first idea was that Lightning had a rolled up map on top of his trunk, then he kicks it off and shows it to Sally.
The message in "Cars" is simplicity itself: Life was better in the old days, when it revolved around small towns where everybody knew each other, and around small highways like Route 66, where you made new friends, sometimes even between Flagstaff and Winona.
It is implied that Doc died before the events of Cars 2, and in Cars 3 he appears in flashback sequences. Doc is voiced by Paul Newman in Cars, Cars 3 (the latter via archive audio recordings) and Cars: the Video Game, and Corey Burton in all other media.
He temporarily took a break from racing and he trained Cruz Ramirez for sometime. After Cruz became good at racing, McQueen continued, but it is unknown when McQueen continued racing. He still races on the Piston Cup circuits today and is winning most of his races.
Fortunately for us, Cars 3 is the real Cars 2. Prior to seeing Cars 3, I was personally excited to see further adventures of race car Lightning McQueen and his pals.
Lightning McQueen is tied with the King for the most cups, also having 7, and Storm won a title during the events of Cars 3. Lightning McQueen best known for competing with the Rust-eze team in the Cup Series from 2005 to 2017 winning 7 Piston Cups in 2006 to 2007, then 2009 to 2010, then 2012 to 2013 and in 2015.
As of now, five characters have known to have won the Piston Cup. Doc Hudson won three titles, Chick Hicks and Jackson Storm have won one title each, and Lightning McQueen and The King are tied for the most wins of any known Cup-winning character with seven each.
