Here's why:
* Entei is part of the Legendary Beasts trio (Raikou, Entei, and Suicune), which are not found in Soul Silver. They were featured in Pokemon Gold and Silver, and their counterparts (Raikou, Entei, and Suicune) appear in HeartGold and SoulSilver.
* The Legendary Beasts are instead replaced by the Legendary Dogs (Raikou, Entei, and Suicune). These dogs have a different appearance and abilities compared to their counterparts.
To catch the Legendary Dogs in Pokemon Soul Silver:
1. Complete the game.
2. Visit the Ruins of Alph. You'll find them in the west of Ecruteak City.
3. Speak to the old man at the ruins. This will start the quest to catch the dogs.
4. Talk to the old man again after the next day. This will trigger the appearance of the dogs. You'll be able to encounter one of them at random:
* Raikou: Found on Route 36, near the Goldenrod City entrance.
* Entei: Found on Route 42, near the Blackthorn City entrance.
* Suicune: Found on Route 47, near the Olivine City entrance.
Remember that the Legendary Dogs will only appear once, so make sure to have your best Poke Balls ready!