Why are the PV of future liabilities not included in the EV to equity bridge?

Dear WSO community,

I was wondering if someone knew the answer to the following question, please:

Why do we only use current market values for non-equity claims when calculating enterprise value when we do trading comparables? We also only use current market values when we back into equity value from enterprise value when we do a DCF valuation.

Why don't we need to discount future liabilities which arise, to today's date? If we know future debt will rise to support the cash flows that are forecast, then why do we not discount future debt to today's value and include it as a non-equity claim? 

The same is true with an item such as pension deficit. We all know to include it in the equity value to enterprise value bridge, but this deficit will almost certainly rise in the future, as the company grows.

Moreover, equity value i.e. market cap is the PV of cash flow into perpetuity attributable to equity investors, so why do we not use PV of non-equity claims into perpetuity as well? Are we not undervaluing equity by only using current claims?

Apologies if I am being dense, but I can't find the answer to this in any finance textbook, and there must be a simple explanation to this...

Thanks in advance to people that can clear this up for me.

Cheers