You can increase reps, but beyond 15-20 reps, you're in cardio land for the most part and strength increases aren't likely or slow to come.
To your question, the point of strength training is to increase muscle mass (preserve it). If you can lift 10 pounds today, your body has enough muscle and doesn't need to make more. The body is efficient. So continuing to lift the same weight over and over doesn't provide any stimulus for growth.
Dumbbells and plates are cheap. You can pick them up used for probably 20-25 bucks a piece. Plates used are less than a dollar a pound. Hardly seems like a king's ransom.