Standing to the left of it would leave the offstump open for being bowled by swing bowlers and spin bowlers. It also becomes very difficult to play a lot of shots and would have a greater risk of edging the ball into the slips.
Slips are fielders behind the stumps (in line with the wicket keeper) which catch edged balls for the batsman to get caught out.
For someone to be LBW the ball has to both pitch (bounce) in line with the stumps and then go on to hit the stumps. The umpire has to rule it definitively. A lot of batsman will pad (knock it away with their pads) the ball away if it bounced outside the line of the stumps but is too dangerous to play. Most batsman will just block or leave a dangerous ball, and only score runs off of worse balls.
It should be noted that bowlers don't often go for the stumps at an international level. There's a lot of trickery and straight balls tend to be easily put away by batsman. Most generally bounce the ball outside of the off-stump (offstump = the stump furthest away from the batsman) and then use swing or spin to either swing/spin back in to bowl or swing/spin back out for them to edge the ball into the slips. Straight balls tend to be surprise balls after a period of bowling regularly.
Swing example of a swing ball. Jones bounces the ball outside of the offstump, tricking the batsman into believing it won't go anywhere near (hence why he "leaves" it). However at the last minute it swings back in and hits his stump. Outward swing example The first is an example of a more regular ball, as you can see the bowler pitches it outside off to swing away. The area of where it bounces encourages the batsman to try and play the ball, but it instead it swings away and makes him edge it into the slips. Spin example Isn't really a point to this, other than to show how pace and spin bowlers differ. Spin bowlers tend to bowl 45-60 mph and use a lot of trickery, like different types of spin, straight balls etc. Pace bowlers tend to bowl 80-90 mph (some going up to 90-100 mph) and will use things like bouncers, swing, yorkers (bounce at the batsman's feet), slower balls in order to intimidate and make the batsman uncomfortable.