Because menopause is a natural life stage for women and hormones change at this stage of life. This does NOT necessarily lead to osteoporosis and heart disease; please don't spread scare stories.
From the NICE guidelines you refer to:
'Osteoporosis—HRT reduces the risk of osteoporotic fracture, the
benefit being maintained while HRT is taken.'
Women will have different risk factors for osteoporosis for a range of factors.
'Cardiovascular disease--no increased risk when started under age
of 60. Appears to be a small increased risk of stroke with tablet but
not transdermal HRT but baseline risk in women under age of 60 is
very small. Studies have shown that starting HRT before the age of
60, or within 10 years of the menopause may reduce the risk
of heart disease, but evidence so far is not strong enough to
confirm this.'
(my bold).