Ok... I read everyone the only way to build muscle is through strength training.
So, can someone explain to me how come some bikers or runners, who only does cardio, get some nice muscle in their legs?
I knew few of them.

It's hard to give a specific answer to your question as we have been given no information on the people you are giving for examples. Starting muscle mass, genetics, definition of "only cardio", whether you are talking about visible muscle mass vs actual muscle mass (a function of body fat levels) etc would all factor into the answer.

However, a few points are:
1. Type II fibers have a greater capacity for hypertrophy than Type I fibers.

2. Anaerobic training utilizes type II fibers to a much higher degree than aerobic training which predominantly relies on Type I fibers which adapt mostly by increasing the number of mitochondria in the cell to allow for greater efficiency in utilizing oxygen.

3. Resistance training, depending upon the set/rep scheme) is predominantly anaerobic. However, not all anaerobic training is resistance training.

4. Not all running and cycling is aerobic although it is all usually lumped into the word "cardio".

5. Even those training predominantly in the aerboic zones will drift above their anaerobic thresholds at some point.

6. Most training programs I have seen for competitive cyclists and runners, even those for endurance events like triathalons and marathons, include anaerobic training. It may not always be resistance training, it may be sprints, shorter distance tempo runs, etc, but they usually include a healthy dose of anaerobic training.

7. There are very few absolutes in life and that applies to training as well. The answer to 99% of the questions is "it depends."

I was reading a blog for a while of a professional bicyclist and a big part of her training was doing weight training. If you look at running training programs, they usually include weight training.

Every distance runner I know personally lifts weights...Muscle appearance, hypertrophy, and strength is dependant on multiple factors---gender, hormones, age, training program, nutrition, body fat%, etc.

As an N of 1, when I started building mileage last fall for my half marathon, my weight went down. Body fat stayed the same, I lost lean mass. I'm sure I appeared as muscular to the naked eye, but the muscle was a little less. I didn't notice any difference in strength.

I'm starting to build mileage again for a full marathon this fall and I am taking steps to preserve my muscle mass as best as I can, with changes in nutrition and also in my weight training program. Even with my runs, I try to limit the steady state running, but at some point, as my miles rack up, steady state happens. Not 100%, there are hills, ups and downs, pauses for traffic, a faster pace with a great song, ducking branches, leaping snakes & cacti....

Don't know what my point is really....

I don't have a clue, and I ask that myself too, but though I think it's not that logical, I have seen it on myself, so maybe in some way I should say it's right, lol. I've done swimming competitions for years, swimming is cardio and I didn't do any weights, my results were medium-low (more low, lol), I was training 3 times per week and on a controlled diet, I didn't drop a pound, I was fat the first day and was the same the last one.
For about one year I did weight training once per week at school, it wasn't he right frequency, but I remember bulding up muscle and loosing weight, even though I was exercizing really less than when I was swimming.
So it doesn't sound that logical to me, but ****, it happened to me!!

Also, keep in mind that everyone has muscles, which appear more or less depending on fat coverage. Running is a big time fat burner, so serious runners tend to have very low body fat percentages comparative to the rest of the population. So the muscle that they do have, even if they aren't focusing on building it specifically, will show more since it has less fat to cover it up.

I am a runner (3-5 miles) and have definitely built muscles in my legs without weight training. I would like to do weight training but don't have access to a gym and don't have anything at home. The only thing in addition to the running I have done is Jillian M. 30 DS which does incorporate some lunge type exercises--maybe this combined with the running has helped.

My thighs aren't buff by any stretch but when I run you can definitely feel and see the muscles in my legs.