I think this is entirely dependent on the person. My partner is just not big on using the phrase ‘I love you’. He doesn’t come from a family that says it often. I’m someone who says it often – I think more because my own insecurity causes me to need to hear it reciprocated.
My dad is the same way as my partner – he mostly says ‘I love you’ in response to someone else saying it. That doesn’t mean he doesn’t Love, it just means he doesn’t feel the need to say it.
Both of these men in my life love me immensely; of that I am certain. They express their love in a multitude of ways. They just aren’t ‘words of affirmation’ people, so they struggle to remember that some people need/want to hear it more often than they might feel the need to say it.