For thinking DP's DM is strange for never setting foot in our home?

[Strongcoffeeneeded]

NC for this as a handful of people know my regular username and knows me in RL.

I've been with my DP for 5 years, been living together for 3. I get on great with DP's DM, been invited to her house regularly and always made to feel very welcome, been invited to family events, holidays with them and she always buys myself and my 3 DC (from previous relationship) wonderful gifts for xmas and birthdays and even when she's been away on holiday, we've had great conversations together and she's just a genuinely caring, loving person and mother. But she has never set foot in the home I share with her son.
She's been invited on several occasions and I think I've always made it very clear she is always welcomed. She only lives a 50 minute drive away and is a very confident driver so I know that's not the issue.
A few months back DP asked her to drop off some important documents she'd found that belonged to him. He told her I was home all day and I waited for her to call round but instead of knocking of even just walking in she posted them through the letter box.
Just last month we were having a party, we sent her a lovely message telling how much we would love to have her company and she never came which upset me a little.
I've asked DP to have a word and ask why, but he wont as he doesn't want to to upset and pressure her. He says she just likes her own company and prefers to be at home which I can understand but at the same time, we've been in this house for 3 years! I can't imagine not visiting my DC in their homes.
His DF has visited us on several occasions (divorced from his DM) and lives 6 hours away so I know DP isn't keeping his parents away.
My friends think I should count my blessings as their MIL's are always 'round their homes, interfering. AIBU for thinking its strange and should I say something or should I just accept this what she wants?

@Strongcoffeeneeded You just don't know, do you? And as long as they're 'nice' I guess there's really not much you can do. It's really kind of a 'no win' situation. I don't think you talking to her would do any good and obviously your DP isn't going to talk to her.

I think that even if I was a 'homebody' who 'never visited anyone' I'd still make a point of visiting my son's home, even if I only accepted every 3rd or 4th invitation.

It is strange, but only to the extent that everyone has their oddities. If you generally get on well then I would put it down to some kind of social awkwardness dressed up as not wanting to be a bother.

It could be nice for your DP to ask her, perhaps just in the context of inviting her for afternoon tea on a weekend and if she says no, explain that you've been wondering why she seems not to want to come over, and has never once been in the house.

It is strange but my grandmother was like that - she loved us dearly but never once set foot in our home. Never did find out why. We spent plenty of time with her, but only at her house.

Well it sounds like it's a hang up of Mil's rather than an issue with your relationship so I would try not to take it personally. Obviously it is odd of her - especially to drive so far and not even stop for a cup of tea. I don't think you'll easily find out why she has this issue.

My PIL don't come here. They are all about family as long as it means they stay at their house and my husband visits them.

Yeah, she passes not far from our home every single Sunday as she spends the day with her Parents so it's not like we're out of her way

It’s possible that she finds spending the day with her parents very tiring and just wants to get home without any further socialising. However, that doesn’t explain why she doesn’t visit other times, especially not when she was dropping the documents off anyway.

Have you ever specifically invited her over for say, Sunday lunch? Or the Gps as well?

I assume she's met your children?

Maybe she feels obliged to visit her parents every weekend and is trying to avoid further obligations.